Process for fabric of continuous graphitic fiber yarns

ABSTRACT

Multi-functional and high-performing fabric comprising a first layer of yarns woven to form the fabric wherein the yarns comprise at least one unitary graphene-based continuous graphitic fiber comprising at least 90% by weight of graphene planes that are chemically bonded with one another having an inter-planar spacing d 002  from 0.3354 nm to 0.4 nm as determined by X-ray diffraction and an oxygen content less than 5% by weight. A majority of the graphene planes in such a continuous graphitic fiber are parallel to one another and parallel to a fiber axis direction. The graphitic fiber contains no core-shell structure, has no helically arranged graphene domains or domain boundaries, and has a porosity level less than 5% by volume, more typically less than 2%, and most typically less than 1% (practically pore-free).

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 13/987,529, entitled “Fabric of continuousgraphitic fiber yarns from living graphene molecules”, filed on Aug. 5,2013, the contents of which are incorporated by reference herein, intheir entirety, for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to the field of graphite fiberyarns and fabrics and, more particularly, to a new class ofmulti-functional yarns and fabrics containing continuous graphiticfibers produced from living graphene molecules or chains. This class ofyarns made up of nearly perfect graphitic fibers exhibits a combinationof exceptionally high tensile strength, elastic modulus, thermalconductivity, electrical conductivity, and ease of functionalizationunmatched by any type of continuous fiber yarns.

BACKGROUND OF THE INVENTION

Continuous carbon fibers and graphite fibers are produced from pitch,polyacrylonitrile (PAN), and rayon. Most carbon fibers (about 90%) aremade from PAN fibers and only a small amount (about 10%) is manufacturedfrom petroleum pitch or rayon. Although the production of carbon fibersfrom different precursors requires different processing conditions, theessential features are similar. Generally, carbon fibers aremanufactured by a controlled pyrolysis of stabilized precursor fibers.Precursor fibers (e.g. PAN) are first stabilized at about 200-400° C. inair by an oxidization process. The resulting infusible, stabilizedfibers are then subjected to a high temperature treatment atapproximately 1,000-1,500° C. (up to 2,000° C. in some cases) in aninert atmosphere to remove hydrogen, oxygen, nitrogen, and othernon-carbon elements. This step is often called carbonization and it cantake 2-24 hours to complete, depending upon the carbonizationtemperature and the starting material used. Carbonized fibers can befurther graphitized at an even higher temperature, up to around 3,000°C. to achieve higher carbon content and higher degree of graphitization,mainly for the purpose of achieving higher Young's modulus or higherstrength in the fiber direction, but not both. This takes another 1-4hours under strictly controlled atmosphere and ultra-high temperatureconditions. The properties of the resulting carbon/graphite fibers areaffected by many factors, such as crystallinity, crystallite sizes,molecular orientation, carbon content, and the type and amount ofdefects.

Specifically, the carbon fibers can be heat-treated to become highmodulus graphite fibers (from pitch) or high strength carbon fibers(from PAN-based). Carbon fibers heated in the range of 1500-2000° C.(carbonization) exhibits the highest tensile strength (5,650 MPa), whilecarbon fiber heated from 2500 to 3000° C. (graphitization) exhibits ahigher modulus of elasticity (531 GPa). The tensile strength ofcarbon/graphite fibers is typically in the range of 1-6 GPa, and theYoung's modulus is typically in the range of 100-588 GPa.

Broadly speaking, in terms of final mechanical properties,carbon/graphite fibers can be roughly classified into ultra-high modulus(>500 GPa), high modulus (>300 GPa), intermediate modulus (>200 GPa),low modulus (100 GPa), and high strength (>4 GPa) carbon fibers. Carbonfibers can also be classified, based on final heat treatmenttemperatures, into type I (2,000° C. heat treatment), type II (1,500° C.heat treatment), and type III (1,000° C. heat treatment). Type IIPAN-based carbon fibers are usually high strength carbon fibers, whilemost of the high modulus carbon fibers belong to type I from pitch.

Regardless the type of carbon fibers or graphite fibers desired, theproduction of continuous carbon fibers and graphite fibers from pitch,PAN, and rayon is a tedious, energy-intensive, very challenging(requiring extreme temperature and atmosphere control), and expensiveprocess. A strong need exists for a facile, less energy-intensive,simpler and more scalable, and more cost-effective process for producingadvanced graphite fibers, yarns, and fabrics.

Carbon is known to have five unique crystalline structures, includingdiamond, fullerene (0-D nano graphitic material), carbon nano-tube orcarbon nano-fiber (1-D nano graphitic material), graphene (2-D nanographitic material), and graphite (3-D graphitic material, includinggraphite fiber). The carbon nano-tube (CNT) refers to a tubularstructure grown with a single wall or multi-wall. Carbon nano-tubes(CNTs) and carbon nano-fibers (CNFs) have a diameter on the order of afew nanometers to a few hundred nanometers. Their longitudinal, hollowstructures impart unique mechanical, electrical and chemical propertiesto the material. The CNT or CNF is a one-dimensional nano carbon or 1-Dnano graphite material. Although multiple CNTs or CNFs can be spun intofiber yarns, these yarns are not considered as “continuous fibers”. Theyare twisted aggregates of individual CNTs or CNFs (each being but a fewmicrons long) that are not self-bonded together; instead, they aremechanically fastened together as a yarn.

Bulk natural graphite is a 3-D graphitic material with each particlebeing composed of multiple grains (a grain being a graphite singlecrystal or crystallite) with grain boundaries (amorphous or defectzones) demarcating neighboring graphite single crystals. Each grain iscomposed of multiple graphene planes that are oriented parallel to oneanother. A graphene plane in a graphite crystallite is composed ofcarbon atoms occupying a two-dimensional, hexagonal lattice. In a givengrain or single crystal, the graphene planes are stacked and bonded viavan der Waal forces in the crystallographic c-direction (perpendicularto the graphene plane or basal plane). Although all the graphene planesin one grain are parallel to one another, typically the graphene planesin one grain and the graphene planes in an adjacent grain are differentin orientation. In other words, the orientations of the various grainsin a graphite particle typically differ from one grain to another.

A graphite single crystal (crystallite) per se is anisotropic with aproperty measured along a direction in the basal plane (crystallographica- or b-axis direction) being dramatically different than if measuredalong the crystallographic c-axis direction (thickness direction). Forinstance, the thermal conductivity of a graphite single crystal can beup to approximately 1,920 W/mK (theoretical) or 1,800 W/mK(experimental) in the basal plane (crystallographic a- and b-axisdirections), but that along the crystallographic c-axis direction isless than 10 W/mK (typically less than 5 W/mK). Further, the multiplegrains or crystallites in a graphite particle are typically all orientedalong different directions. Consequently, a natural graphite particlecomposed of multiple grains of different orientations exhibits anaverage property between these two extremes; i.e. between 5 W/mK and1,800 W/mK.

It would be highly desirable in many applications to produce acontinuous graphitic fiber (containing single or multiple grains) havinga sufficiently large length and having all graphene planes beingessentially parallel to one another along one desired direction (e.g.along the fiber axis). For instance, it is highly desirable to have along graphite fiber (e.g. a fully integrated or unitary filament ofmultiple graphene planes) having all the constituent graphene planesbeing substantially parallel to one another along the fiber axisdirection without forming a helical structure or a porous structure. Itwould be further desirable if such a long or continuous graphite fiberhas only one grain or few grains (thus, no or little grain boundaries)and has few defects therein to impede the flow of electrons and phonons.Preferably, the grain size along the fiber axis direction is larger than100 μm, more preferably mm in dimension, further preferably cm indimension, still further preferably meters in dimension. Thus far, ithas not been possible to produce this type of large-size unitarygraphene entity (fiber) from existing natural or synthetic graphiteparticles. This is part of what we have accomplished in the instantinvention.

The constituent graphene planes of a graphite crystallite in a graphiteparticle can be exfoliated and extracted or isolated from a graphitecrystallite to obtain individual graphene sheets of carbon atomsprovided the inter-planar van der Waals forces can be overcome. Anisolated, individual graphene sheet of carbon atoms is commonly referredto as single-layer graphene. A stack of multiple graphene planes bondedthrough van der Waals forces in the thickness direction with aninter-graphene plane spacing of 0.3354 nm is commonly referred to as amulti-layer graphene. A multi-layer graphene platelet has up to 300layers of graphene planes (<100 nm in thickness), but more typically upto 30 graphene planes (<10 nm in thickness), even more typically up to20 graphene planes (<7 nm in thickness), and most typically up to 10graphene planes (commonly referred to as few-layer graphene inscientific community). Single-layer graphene and multi-layer graphenesheets are collectively called “nano graphene platelets” (NGPs).Graphene sheets/platelets or NGPs are a new class of carbon nanomaterial (a 2-D nano carbon) that is distinct from the 0-D fullerene,the 1-D CNT, and the 3-D graphite.

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A.Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets andNanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25,2006).

NGPs are typically obtained by intercalating natural graphite particleswith a strong acid and/or oxidizing agent to obtain a graphiteintercalation compound (GIC) or graphite oxide (GO), as illustrated inFIG. 1(a) (process flow chart) and FIG. 1(b) (schematic drawing). Thepresence of chemical species or functional groups in the interstitialspaces between graphene planes serves to increase the inter-graphenespacing (d₀₀₂, as determined by X-ray diffraction), therebysignificantly reducing the van der Waals forces that otherwise holdgraphene planes together along the c-axis direction. The GIC or GO ismost often produced by immersing natural graphite powder (20 in FIGS.1(a) and 100 in FIG. 1(b)) in a mixture of sulfuric acid, nitric acid(an oxidizing agent), and another oxidizing agent (e.g. potassiumpermanganate or sodium perchlorate). The resulting GIC (22 or 102) isactually some type of graphite oxide (GO) particles. This GIC is thenrepeatedly washed and rinsed in water to remove excess acids, resultingin a graphite oxide suspension or dispersion, which contains discreteand visually discernible graphite oxide particles dispersed in water.This rinsing step may be followed by several different processingroutes:

For instance, Route 1 involves removing water from the suspension toobtain “expandable graphite,” which is essentially a mass of dried GICor dried graphite oxide particles. Upon exposure of expandable graphiteto a temperature in the range of typically 800-1,050° C. forapproximately 30 seconds to 2 minutes, the GIC undergoes a rapidexpansion by a factor of 30-300 to form “graphite worms” (24 or 104),which are each a collection of exfoliated, but largely un-separatedgraphite flakes that remain interconnected. A SEM image of graphiteworms is presented in FIG. 2(a).

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (26 or 106) that typically havea thickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm). Flexiblegraphite (FG) foils can be used as a heat spreader material, butexhibiting a maximum in-plane thermal conductivity of typically lessthan 500 W/mK (more typically <300 W/mK) and in-plane electricalconductivity no greater than 1,500 S/cm. These low conductivity valuesare a direct result of the many defects, wrinkled or folded graphiteflakes, interruptions or gaps between graphite flakes, and non-parallelflakes (e.g. SEM image in FIG. 2(b)). Many flakes are inclined withrespect to one another at a very large angle (e.g. mis-orientation of20-40 degrees).

Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814.Single-layer graphene can be as thin as 0.34 nm, while multi-layergraphene can have a thickness up to 100 nm, but more typically less than20 nm.

Exfoliated graphite worms, expanded graphite flakes, and therecompressed mass of graphite worms (commonly referred to as flexiblegraphite sheet or flexible graphite foil) are all 3-D graphiticmaterials that are fundamentally different and patently distinct fromeither the 1-D nano carbon material (CNT or CNF) or the 2-D nano carbonmaterial (graphene sheets or platelets, NGPs).

Route 2 entails ultrasonicating the graphite oxide suspension for thepurpose of separating/isolating individual graphene oxide sheets fromgraphite oxide particles. This is based on the notion that theinter-graphene plane separation has been increased from 0.3354 nm innatural graphite to 0.6-1.1 nm in highly oxidized graphite oxide,significantly weakening the van der Waals forces that hold neighboringplanes together. Ultrasonic power can be sufficient to further separategraphene plane sheets to form separated, isolated, or discrete grapheneoxide (GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight and, most typically and desirably, less than 2% byweight.

For the purpose of defining the claims of the instant application, NGPsinclude discrete sheets/platelets of single-layer and multi-layergraphene, graphene oxide, or reduced graphene oxide with an oxygencontent of 0-10% by weight, more typically 0-5% by weight, andpreferably 0-2% by weight. Pristine graphene has essentially 0% oxygen.Graphene oxide (including RGO) can have approximately 0.001%-50% byweight of oxygen.

The GO molecules in graphene oxide gel, to be described in detail later,typically contain 20-50% by weight oxygen (more typically 30-47%)immediately after removal of the liquid from the GO gel, but prior to asubsequent heat treatment. The GO gel refers to a homogeneous solutionof highly hydrophilic aromatic molecules (graphene oxide moleculesbearing oxygen-containing groups, such as —OH, —COOH, and >0, onmolecular planes or at the edges) that are dissolved (not justdispersed) in a liquid (e.g. acidic water). The GO gel per se does notcontain visibly discernible or discrete graphene or GO particles in theform of solid sheets or platelets dispersed in the liquid medium. TheseGO molecules and the dissolving liquid medium have comparable indices ofrefraction, making the resulting gel optically transparent ortranslucent (if the proportion of GO molecules are not excessively high;e.g. <2% GO), or showing lightly brown color. In contrast, the simplemixture of original graphite particles or discrete graphenesheets/platelets with acids and/or water appears optically dark andtotally opaque (even with only <0.1% solid particles suspended in theliquid medium). These particles or NGP platelets are simply dispersed(not dissolved) in the fluid medium.

These GO molecules in a GO gel are highly reactive and may be consideredas “living giant molecules” or “living chains”. By contrast, the priorart solid sheets/platelets of graphene, GO, and RGO are essentially“dead” species. The GO gel can be formed into a shape with a propershearing or compression stress (e.g. via casting or extrusion through atapered-diameter nozzle), dried (with liquid components partially ortotally removed), and heat-treated under certain conditions to obtain aunitary graphene material (e.g. a continuous filament of the instantinvention), which is typically a single crystal, a poly-crystal withincomplete or poorly delineated grain boundaries, or a poly-crystal withvery large grain sizes (very few grains). The heat treatment serves tochemically link these active or living GO molecules to form a 2-D or 3-Dnetwork of chemically bonded graphene molecules of essentially infinitemolecular weights, and to drastically reduce the oxygen content of GOdown to below 10% by weight, more typically <5%, further more typically<2%, and most typically <<1%. Only a trace amount of oxygen (practically0%) can survive if the heat treatment temperature is sufficiently high(>2,500° C.) and heat treatment time sufficiently long. This new andunique material called “unitary graphene material” in a continuousfilament form will be further described in detail later. When in afilamentary form as disclosed herein, this unitary graphene material isa nearly perfect graphitic fiber.

Solid or “dead” NGPs (including discrete sheets/platelets of pristinegraphene, GO, and GRO), when packed into a film, membrane, or papersheet (34 or 114) of non-woven aggregates, typically do not exhibit ahigh thermal conductivity unless these sheets/platelets are closelypacked and the film/membrane/paper is ultra-thin (e.g. <1 μm, which ismechanically weak). This is reported in our earlier U.S. patentapplication Ser. No. 11/784,606 (Apr. 9, 2007). In general, a paper-likestructure or mat made from platelets/sheets of graphene, GO, or RGO(e.g. those paper sheets prepared by vacuum-assisted filtration process)exhibit many defects, wrinkled or folded graphene sheets, interruptionsor gaps between platelets, and non-parallel platelets (e.g. SEM image inFIG. 3(b)), leading to relatively poor thermal conductivity, lowelectric conductivity, and low structural strength.

In a recent report [Z. Xu & C. Gao, “Graphene chiral liquid crystals andmacroscopic assembled fibers,” Nature Communications, 2, 571 (2011)],graphene oxide sheets can form chiral liquid crystals in atwist-grain-boundary phase-like model with simultaneous lamellarordering and long-range helical frustrations. Aqueous graphene oxideliquid crystals can then be continuously spun into meters of macroscopicgraphene oxide fibers, which are chemically reduced to obtain RGOfibers. During the spinning process for GO fibers, the GO dispersionswere loaded into glass syringes and injected into the NaOH/methanolsolution under the conditions of 1.5 MPa N₂. The NaOH/methanol solutionis a coagulation solution (a non-solvent for GO) and the GO sheets areprecipitated out as discrete/isolated sheets that are mechanicalfastened in the fiber form as soon as the GO dispersions came in contactwith the non-solvent in a coagulation bath. The fibers produced in thecoagulation bath were then rolled onto a drum, washed by methanol toremove the salt, and dried for 24 hours at room temperature. Theas-prepared GO fibers were then chemically reduced in the aqueoussolution of hydro-iodic acid (40%) at 80° C. for 8 hours, followed bywashing with methanol and vacuum drying for 12 hours.

Clearly, this is a very tedious and time-consuming process. Further, theGO sheets must be dispersed in water to a critical extent that they formchiral liquid crystals with a twist-grain-boundary phase structure inthe GO suspension. This chiral or twist-grain boundary structure is afatal defect as far as the mechanical strength of macroscopic graphenefibers is concerned, as evidenced by the relatively low tensile strength(102 MPa) reported by Xu, et al. This is three orders of magnitude lowerthan the intrinsic strength (130 GPa) of individual graphene sheets.Another severe problem of this process is the notion that thespinning-coagulation procedure inherently results in highly porous andnon-oriented graphene sheets in the graphene fiber (e.g. FIG. 2(c) andFIG. 2(d)). This porous and non-parallel graphene structure is anotherreason responsible for such a low tensile strength and low Young'smodulus (5.4 GPa), which is almost three orders of magnitude lower thanthe theoretical Young's modulus of graphene (1,000 GPa).

A similar spinning-coagulation process was reported by Cong, et al [H.P. Cong, et al. “Wet-spinning assembly of continuous, neat, andmacroscopic graphene fibers,” Scientific Report, 2 (2012) 613; DOI:10.1038/srep00613]. Again, the reported tensile strength and Young'smodulus of the graphene fibers are very poor: 145 MPa and 4.2 GPa,respectively. Slightly better tensile strength (180 MPa) was observedwith graphene oxide fibers prepared by a confined-dimension hydrothermalmethod was reported [Z. Dong, et al. “Facile fabrication of light,flexible and multifunctional graphene fibers,” Adv. Mater. 24, 1856-1861(2012)]. Even after a thermal reduction treatment, the maximumachievable tensile strength was only 420 MPa. Again, the graphene sheetsin these graphene fibers, just like in the graphene fibers prepared byspinning-coagulation, remain discrete and poorly oriented. The fibersare also highly porous and of limited length. Furthermore, this processis not a scalable process and cannot be used to mass-produce continuousgraphene fibers.

In most of the practical applications, fibers and yarns are not thefinal utilization shape or form. A particularly useful form is fabric,which is obtained by weaving yarns of fibers. The properties of a fabricdepend on the properties of the fibers. For illustration purposes,cotton or wool fibers are used to keep a person warm in the winter,asbestos fibers are used as a flame retardant, carbon fibers forstrength reinforcement, glass fibers for insulation, metallic fibers forconducting electricity. Unfortunately, combining fibers does not alwaysresult in a fabric that possesses a useful set of properties for a rangeof applications. For example, anti-ballistic fibers, such as Kevlar, aresensitive to heat. Although adding flame retardant fibers may providelimited support, Kevlar fabrics would not work optimally as a projectileresistant material if exposed to continuous heat. Ideally, compatiblefibers having unique mechanical, thermal, electrical, optical, andchemical properties would be woven into fabrics that demonstrate all thedesired properties within the fabric. However, all the state-of-the-artfabrics have a limited range of applications due to the limitedfunctional properties of their constituent fibers.

In addition, fabric quality and functional performance depends on theability to inter-weave yarns with one another. The material structure,size, and shape of the fibers and resulting yarns may become limitingfactors for the range of application of a certain fabric. For examples,fabrics that block entry of pathogenic agents require that the yarns ofconsistent quality be interwoven tightly to prevent any gaps between oneanother. The thickness and shapes of individual fibers alone could allowsignificant gaps within each yarn defined by those fibers. Generally,there are no available continuous fibers having a nanometerdiameter/thickness and shape that provide significant strength,ductility, geometric flexibility, and cross-sectional shape of a yarn soas to define a multi-functional fabric. There is an urgent need to havea new type of graphitic fibers that can be made into a multi-functionalfabric.

Our recent patent applications have provided a process for producinghigh-strength and high-modulus continuous graphitic fibers by usingparticles of natural graphite or artificial graphite as the startingmaterial. Please refer to: A. Zhamu and B. Z. Jang, “ContinuousGraphitic Fibers from Living Graphene Molecules,” U.S. patentapplication Ser. No. 13/986,223 (Apr. 15, 2013) and “Process forProducing Continuous Graphitic Fibers from Living Graphene Molecules,”U.S. patent application Ser. No. 13/986,208 (Apr. 15, 2013).Specifically, these applications have provided a graphene oxidegel-derived continuous graphitic fiber that is a unitary graphenematerial or monolithic graphene entity, not just an aggregate ofdiscrete graphene or graphene oxide sheets. The GO gel-derived unitarygraphene filaments exhibit a combination of exceptional thermalconductivity, electrical conductivity, mechanical strength, and elasticmodulus unmatched by any continuous graphene fibers or carbon fibers.Specifically, these highly conductive, continuous graphitic fibersexhibit the following properties: (a) a thermal conductivity greaterthan 600 W/mK (typically greater than 1,000 W/mK, and can be greaterthan 1,700 W/mK); (b) an electrical conductivity greater than 2,000 S/cm(typically >3,000 S/cm, more typically >5,000 S/cm, often >10,000 S/cm,and even >15,000 S/cm); (c) a tensile strength greater than 1.2 GPa(typically >3.2 GPa, more typically >5.0 GPa, and can be >8.0 GPa);and/or (d) a Young's modulus greater than 60 GPa (typically >200 GPa,more typically >300 GPa, and often >600 GPa). No prior art continuousgraphitic fiber meets this set of stringent technical requirements.

These exceptional properties of our continuous graphitic fibers areproduced from living graphene chains by a unique and novel processwithout following the coagulation-spinning procedure or spinning fromCVD graphene films.

We proceeded to further investigate the technical feasibility of weavingthese continuous graphitic fibers into a fabric and explore thepotential utilization of such a fabric. Through this investigation wehave made several surprising observations and inventions. These newgraphene fibers are generally flat-shaped in cross-section(non-circular, non-ellipsoidal, and non-oval shape), with a large width(typically from 0.01 μm to 20 μm and more typically from 0.1 μm to 10μm, but readily adjustable) and a small thickness (typically from 1 nmto 1 μm, readily adjustable), hence a high width-to-thickness ratio(typically from 10 to 1000). They are relatively solid, non-porous.These shapes, structures, and morphologies are in contrast to thosegraphene fibers produced by coagulation and spinning, which are helicaland highly porous in nature and having a chiral or twist-grain boundarystructure. The helical structure and high porosity level of theseconventional graphene fibers are a natural consequence of the liquidcrystal structure of the starting graphene oxide material and therequired precipitation of graphene from a liquid coagulation bath.Additionally, the graphene fibers obtained by drawing CVD graphene filmsinto a fibrous form are also highly porous. These pores and helicesseverely weaken these conventional fibers, exhibiting dramatically lowerelastic modulus and strength.

We have further observed that, due to the more or less rectangularcross-section of the presently invented continuous graphitic fibers, theyarns containing multiple continuous fibers can have a cross-sectionthat is rectangular or flat-shaped. When one combines multiple filamentstogether (e.g. of those conventional fibers with a circularcross-section or irregular-shape cross-section), there is a limit to thepacking factor. The highest packing factor is typically between 50% and65% by volume even for circular-cross-section fibers. In contrast, thepresently invented rectangular or flat-shaped graphene fibers can bepacked into a yarn with an essentially 100% packing factor. The packingfactor can be adjusted to be between 20% and essentially 100%, forcomposite structure or filter applications. A packing factor of 70-85%is particularly useful for composite applications. Our research datahave demonstrated that the flexural strength and elastic modulus valuesof polymer matrix composites containing presently invented graphiticfiber-based fabrics as a reinforcement phase are significantly higherthan those of the composites containing a comparable volume fraction ofconventional graphitic fibers. Additionally, fabrics that block entry ofpathogenic agents require that the yarns of highest packing factors beinterwoven tightly to prevent any gaps between one another. Thethickness and shapes of conventional fibers alone could allowsignificant gaps within each yarn defined by those fibers. The instantinvention provides tightly packed yarns and fabrics. These features arenot achievable with conventional graphitic fibers.

SUMMARY OF THE INVENTION

An embodiment of the present invention is a fabric comprising a firstlayer of yarns woven to form the fabric wherein the yarns comprise atleast one unitary graphene-based continuous graphitic fiber comprisingat least 90% by weight of graphene planes that are chemically bondedwith one another having an inter-planar spacing d₀₀₂ from 0.3354 nm to0.4 nm as determined by X-ray diffraction and an oxygen content lessthan 5% by weight (typically from 0.001% to 5% by weight), wherein thegraphene planes are parallel to one another and parallel to a fiber axisdirection and the graphitic fiber contains no core-shell structure, hasno helically arranged graphene domains or domain boundary, and has aporosity level less than 10% by volume (more typically <5%). In apreferred embodiment, the inter-plane spacing d₀₀₂ is from 0.3354 nm to0.36 nm, the oxygen content is less than 2% by weight, and/or porositylevel is less than 2% by volume.

One interesting and unique characteristic of the presently inventedfabric is that the constituent fibers derived from living graphenechains can be made into a more or less rectangular cross-section. As aconsequence, the yarns containing multiple continuous fibers can have across-section that is rectangular or flat-shaped. The fibers can becombined into a yarn having a packing factor >60% by volume (voidcontent <40% by volume). The packing factor can be and typically isgreater than 70% or even 80%. In principle, the rectangular fibers ofthe instant invention enable a yarn packing factor approaching 100% byvolume. Preferably, the yarns have a width-to-thickness ratio greaterthan 5, more preferably >20, and can be greater than 150. The fabric orthe yarn can have a thickness less than 1 μm, or even less than 100 nm.

In a further preferred embodiment, the continuous graphitic fiber in thefabric has an oxygen content less than 1%, an inter-graphene spacingless than 0.345 nm, a thermal conductivity of at least 1,000 W/mK,and/or an electrical conductivity no less than 3,000 S/cm. Furtherpreferably, the continuous graphitic fiber has an oxygen content lessthan 0.01%, an inter-graphene spacing less than 0.337 nm, a thermalconductivity of at least 1,200 W/mK, and/or an electrical conductivityno less than 5,000 S/cm. Still further preferably, the continuousgraphitic fiber has an oxygen content no greater than 0.001%, aninter-graphene spacing less than 0.336 nm, a mosaic spread value nogreater than 0.7, a thermal conductivity of at least 1,500 W/mK, and/oran electrical conductivity no less than 8,000 S/cm. The continuousgraphitic fiber can have an inter-graphene spacing less than 0.336 nm, amosaic spread value no greater than 0.4, a thermal conductivity greaterthan 1,700 W/mK, and/or an electrical conductivity greater than 12,000S/cm.

In a preferred embodiment, the continuous graphitic fiber has aninter-graphene spacing less than 0.337 nm and a mosaic spread value lessthan 1.0. In a further preferred embodiment, the continuous graphiticfiber has a degree of graphitization no less than 40% and/or a mosaicspread value less than 0.7. Most preferably, the continuous graphiticfiber has a degree of graphitization no less than 80% and/or a mosaicspread value no greater than 0.4.

In a relaxed or un-stressed state, the continuous graphitic fibercontains chemically bonded graphene molecules or chemically mergedgraphene planes that are parallel to one another and are parallel to thefiber axis direction. Along the fiber axis direction, the grapheneplanes are not helically arranged. In such a non-helical conformation,the continuous graphitic fiber contains a first graphene domaincontaining bonded graphene planes parallel to one another and having afirst crystallographic c-axis, and a second graphene domain containingbonded graphene planes parallel to one another and having a secondcrystallographic c-axis wherein the first crystallographic c-axis andthe second crystallographic c-axis are inclined with respect to eachother at an angle less than 10 degrees.

In an embodiment of the present invention, the continuous graphiticfiber in the fabric contains a poly-crystal graphite structure withgraphene molecules being oriented along a fiber axis direction. Thecontinuous graphitic fiber can have a poly-crystalline graphiticstructure having a grain size larger than 1 μm, more commonly largerthan 10 μm, and most commonly larger than 100 μm. In many cases, thegrains are larger than one centimeter. There are no other graphene-basedmaterials that contain grains larger than a few μm. This implies thatthe presently invented continuous graphitic fibers are relativelydefect-free and are, practically speaking, perfect graphite crystals.

The continuous graphitic fiber typically has an electrical conductivitygreater than 3,000 S/cm, a thermal conductivity greater than 600 W/mK, aphysical density greater than 1.7 g/cm³, a Young's modulus greater than60 GPa, and/or a tensile strength greater than 1.2 GPa. It is morecommon that the continuous graphitic fiber has an electricalconductivity greater than 5,000 S/cm, a thermal conductivity greaterthan 1,000 W/mK, a physical density greater than 1.8 g/cm³, a Young'smodulus greater than 200 GPa, and/or a tensile strength greater than 3.2GPa. Further typically, the continuous graphitic fiber has an electricalconductivity greater than 15,000 S/cm, a thermal conductivity greaterthan 1,500 W/mK, a physical density greater than 1.9 g/cm³, a Young'smodulus greater than 300 GPa, and/or a tensile strength greater than 5.0GPa. A well-prepared continuous graphitic fiber has an electricalconductivity greater than 18,000 S/cm, a thermal conductivity greaterthan 1,700 W/mK, a physical density greater than 1.9 g/cm³, a Young'smodulus greater than 600 GPa, and/or a tensile strength greater than 8.0GPa.

In a preferred embodiment, the yarns in the fabric comprise at least onefiber selected from the group consisting of wool, cotton, asbestos,nylon, synthetic, carbon nanotubes, and graphene-based graphitic fiber.The first layer of yarns woven together normally exhibit the physical,electrical, mechanical, chemical, or thermal properties of the unitarygraphene-based continuous graphitic fibers that constitute these yarns.The fabric can comprise at least one additional layer of yarns woventogether. This additional layer of yarns further comprises a unitarygraphene-based continuous graphitic fiber.

Preferably the fabric is a woven fabric comprising a flat yarn ofgraphene-derived graphitic fibers as at least its warp or weft. The flatyarn is preferably twist-free and the ratio of yarn width to yarnthickness is from 10 to 150.

A preferred process for producing such a continuous graphitic fibercomprises: (a) preparing a graphene oxide gel having living grapheneoxide molecules or functionalized graphene chains dissolved in a fluidmedium; (b) depositing at least a continuous filament of graphene oxidegel onto a supporting substrate under a condition of stress-inducedmolecular alignment of living graphene oxide molecules along a filamentaxis direction; (c) removing the fluid medium to form a continuousgraphene oxide fiber, having an inter-plane spacing d₀₀₂ of 0.4 nm to1.2 nm and an oxygen content no less than 5% by weight; and (d) heattreating the continuous graphene oxide fiber to form the continuousgraphitic fiber at a temperature higher than 600° C. (preferably >1,000°C.) to an extent that an inter-plane spacing d₀₀₂ is decreased to avalue of 0.3354-0.36 nm and the oxygen content is decreased to less than5% by weight.

The unitary graphene-based continuous graphitic fiber containschemically bonded graphene molecules or chemically merged grapheneplanes that are parallel to one another. Typically, the continuousgraphitic fiber contains no complete grain boundary therein, is agraphite single crystal, or a poly-crystal graphite structure withgraphene molecules being oriented along a fiber axis direction. Thecontinuous graphitic fiber can be a poly-crystal graphitic structurehaving a grain size larger than 1 μm, preferably and typically largerthan 10 μm, even more preferably and typically larger than 100 μm. Theunitary graphene-based continuous graphitic fiber contains a combinationof sp² and sp³ electronic configurations if the final heat treatmenttemperature is significantly lower than 2,000° C. Above a HTT of 2,000°C., most of the bonding in the presently invented graphitic fiberappears to be sp² on graphene plane and van der Waals forces betweengraphene planes.

The present invention also provides a process for producing a continuousgraphitic fiber from living graphene molecules, including graphene oxideand functionalized graphene molecules capable of chemically self-linkingor bonding with one another (not just mechanical fastening orinterlocking). The process comprises: (a) preparing a graphene oxide gelhaving living graphene oxide molecules or functionalized graphene chainsdissolved in a fluid medium wherein the graphene oxide molecules containan oxygen content higher than 10% by weight; (b) dispensing anddepositing at least a continuous filament of graphene oxide gel onto asupporting substrate, wherein the dispensing and depositing procedureincludes mechanical stress-induced molecular alignment of the livinggraphene oxide molecules or functionalized graphene chains along afilament axis direction; (c) partially or completely removing the fluidmedium from said continuous filament to form a continuous graphene oxidefiber, wherein the graphene oxide fiber has an inter-plane spacing d₀₀₂of 0.4 nm to 1.2 nm as determined by X-ray diffraction and an oxygencontent no less than 10% by weight; and (d) heat treating the continuousgraphene oxide fiber to form the continuous graphitic fiber at a heattreatment temperature higher than 100° C. (preferably >600° C. and morepreferably >1,000° C.) to the extent that an inter-plane spacing d₀₀₂ isdecreased to a value of from 0.3354 nm to 0.4 nm and the oxygen contentis decreased to less than 5% by weight (preferably <1%). Multiplegraphene fibers thus produced can be combined to form a continuous fiberyarn.

In one preferred embodiment, step (c) includes forming a continuousgraphene oxide fiber having an inter-plane spacing d₀₀₂ of 0.4 nm to 0.7nm and an oxygen content no less than 10% by weight; and step (d)includes heat-treating the continuous graphene oxide fiber to an extentthat an inter-plane spacing d₀₀₂ is decreased to a value of from 0.3354nm to 0.36 nm and the oxygen content is decreased to less than 2% byweight.

In a preferred embodiment, the procedure of mechanical stress-inducedmolecular alignment includes shear-induced thinning of the grapheneoxide gel. The graphene oxide gel preferably has a viscosity greaterthan 2,000 centipoise when measured at 20° C. prior to shear-inducedthinning, and the viscosity is reduced to less than 2,000 centipoiseduring or after shear-induced thinning. In general, the graphene oxidegel has a viscosity from 500 centipoise to 500,000 centipoise whenmeasured at 20° C. prior to the procedure of mechanical stress-inducedmolecular alignment. The viscosity is reduced to less than 2,000centipoise during or after shear-induced thinning. Typically, theviscosity is decreased by at least 10 times when a shear rate isincreased at 20° C. by a factor of 10. The procedure of mechanicalstress-induced molecular alignment may be conducted via a procedureselected from coating, casting, injection, extrusion, pultrusion, orspinning of the graphene oxide gel onto a solid substrate along a fiberaxis direction.

The procedure of mechanical stress-induced molecular alignment caninvolve a shear stress. Shear-induced thinning in step (b) means the GOgel is subjected to a shear stress during processing and a viscosity ofthe GO gel is reduced during and/or after the application of such ashear stress. As an example, the shear stress can be encountered in asituation where the GO gel is being extruded from an extrusion die slitthat has a larger inner diameter (at a distance from the exit) graduallytapered to a smaller inner diameter at the exit point. As anotherexample, an effective shear stress is created when a stream of GO gel isdispensed from a nozzle to a moving solid substrate, such as a plasticfilm, where the gap between the nozzle and the moving substrate can bereduced to induce a stronger shearing effect. In contrast, conventionalspinning-coagulation processes allow the extruded strands of polymerchains to relax out when brought in contact with the coagulation liquid.

In another embodiment, step (d) includes heat treating the continuousgraphene oxide fiber under a stress field that includes a local tensionstress along a fiber axis direction. This tension force exerted on theGO fiber helps to maintain or even enhance the molecular orientation ofthe fiber during a heat treatment.

The continuous graphitic fiber can have a cross-section that iscircular, elliptical, rectangular, flat-shaped, or hollow depending uponthe geometry of the shaping die used. Preferred shapes of continuousgraphitic fibers for use in the fabric are rectangular or flat-shaped.The diameter of the presently invented graphitic fiber can be variedfrom nanometer scaled to millimeter-scaled; there is no restriction onthe fiber diameter. This is a very important feature that cannot befound in any other type of continuous carbon fiber or graphite fiber.

For instance, the GO gel-derived continuous graphitic fiber can have adiameter or thickness up to 100 μm (or greater), which cannot beobtained with conventional carbon or graphite fibers. The continuousgraphitic fiber can have a diameter or thickness less than 10 μm or evenless than 1 μm, which is not possible with other types of continuouscarbon or graphite fibers having a high strength. Quite significantly,the continuous graphitic fiber can have a diameter or thickness lessthan 100 nm.

The mechanical stress-induced molecular alignment (e.g. viashear-induced thinning) is a critically important step in the productionof the presently invented unitary graphene-based graphitic fibers due tothe surprising observation that shear-induced thinning during GO geldispensing and deposition onto a solid substrate (as opposed to a liquidcoagulation bath) enables the GO molecules to align themselves along aparticular direction (e.g. the fiber-axis direction) to achieve apreferred orientation. Further surprisingly, this preferred orientationof graphene molecules is preserved and often further enhanced during thesubsequent heat treatment to produce the unitary graphene-basedgraphitic fiber. Most surprisingly, such a preferred orientation isessential to the eventual attainment of exceptionally high thermalconductivity, high electrical conductivity, high tensile strength, andhigh Young's modulus of the resulting unitary graphene fiber along thefiber axis direction. These great properties in this desired directioncould not be obtained without such a mechanical stress-inducedorientation control.

In one embodiment, the graphene oxide gel is obtained by immersingpowders or filaments of a graphitic material in an oxidizing liquidmedium (e.g. a mixture of sulfuric acid, nitric acid, and potassiumpermanganate) in a reaction vessel. The starting graphitic material maybe selected from natural graphite, artificial graphite, meso-phasecarbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hardcarbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof. When the graphite powders or filaments are mixed inthe oxidizing liquid medium, the resulting slurry initially appearscompletely dark and opaque. The resulting mass is simply a heterogeneoussuspension of solid particles dispersed (not dissolved) in a liquidmedium. When the oxidation of graphite proceeds at a reactiontemperature for a sufficient length of time under a controlled pHcondition, the reacting mass can eventually turn optically translucent,transparent (if sufficiently dilute), or uniform brown color which alsolooks and behaves like a gel. This heavy oxidation-induced grapheneoxide gel is composed of graphene oxide molecules uniformly dissolved inthe liquid medium. We observe that even if the initial solid graphitepowder particles dispersed in water occupy a proportion as low as 0.1%by weight or lower, the initial suspension is heterogeneous and lookscompletely dark and opaque. In contrast, the GO gel is a homogeneoussolution, containing no discernible discrete solid particles. Even whenthe GO molecule content exceeds 1% by weight, the GO gel can appeartranslucent or transparent.

The graphene oxide molecules in the GO gel, prior to any subsequentchemical functionalization or heat treatment, typically have an oxygencontent no less than 10% by weight (more typically greater than 20% byweight, further more typically greater than 30% by weight, and mosttypically from 40-50% by weight) and their molecular weights aretypically less than 43,000 g/mole (often less than 4,000 g/mole, buttypically greater than 200 g/mole) while in a gel state. The grapheneoxide gel is composed of graphene oxide molecules dissolved (not justdispersed) in an acidic medium having a pH value of typically no higherthan 5, more typically lower than 3.

Subsequently, the GO gel is formed into a filamentary shape (e.g.dispensed and deposited on a solid substrate) under the influence ofmechanical stresses (shear stress, in particular). Subsequently, theliquid component in the GO gel is partially or completely removed toobtain an at least partially dried GO filament containing well-packedand well-aligned living GO molecules.

In one embodiment, the graphene oxide molecules in step (a) contain anoxygen content higher than 30% by weight. In another embodiment, step(c) includes forming a graphene oxide filament having an inter-planespacing d₀₀₂ of 0.4 nm to 0.7 nm and an oxygen content no less than 20%by weight; and step (d) includes heat-treating the graphene oxide layerto the extent that the inter-plane spacing d_(o02) is decreased to avalue in the range of 0.3354 nm to 0.36 nm and the oxygen content isdecreased to less than 2% by weight.

In still another embodiment, the graphene oxide gel has a viscositygreater than 2,000 cP (centipoise) when measured at 20° C. prior to theshear-induced thinning procedure, but the viscosity is reduced to below2,000 cP (or even below 1,000 cP) during or after shear-inducedthinning. In still another embodiment, the graphene oxide gel has aviscosity greater than 5,000 cP when measured at 20° C. prior toshear-induced thinning, but is reduced to below 5,000 cps (preferablyand typically below 2,000 cP or even below 1,000 cP) during or aftershear-induced thinning. Preferably, the graphene oxide gel has aviscosity from 500 cP to 500,000 cP when measured at 20° C. prior toshear-induced thinning.

Preferably, the graphene oxide gel has a viscosity less than 5,000 cP(preferably less than 2,000 cP and further preferably less than 1,000cP) when measured at 20° C. after shear-induced thinning. In general,the graphene oxide gel has a viscosity that decreases by at least 10times when a shear rate is increased to a finite extent (e.g. by afactor of 10) at 20° C.

The dried GO filament after deposition is then subjected to a properlyprogrammed heat treatment that can be divided into four distincttemperature regimes. The presently invented unitary graphene-basedgraphitic fiber can be obtained by heat-treating the dried GO filamentwith a temperature program that covers at least the first regime, morecommonly covers the first two regimes, still more commonly the firstthree regimes, and most commonly all the 4 regimes (the latter beingimplemented to achieve the highest electric conductivity, highestthermal conductivity, highest strength, and highest modulus):

-   Regime 1: 100° C.-600° C. (the thermal reduction regime); Oxygen    content reduced from typically 30-50% to 5-6%, resulting in a    reduction of inter-graphene spacing from approximately 0.6-1.0 nm to    approximately 0.4 nm and an increase in the axial thermal    conductivity of a GO filament from approximately 100 to 450 W/mK.-   Regime 2: 600° C.-1,250° C. (the chemical linking regime); Oxygen    content reduced to typically 0.7% (<<1%), resulting in a reduction    of inter-graphene spacing to approximately 0.345 nm, an increase in    axial thermal conductivity of the filament to 1,000-1,200 W/mK,    and/or in-plane electrical conductivity to 2,000-3,000 S/cm.-   Regime 3: 1,250° C.-2,000° C. (the ordering and re-graphitization    regime); Oxygen content reduced to typically 0.01%, resulting in a    reduction of inter-graphene spacing to approximately 0.337 nm    (degree of graphitization from 1% to approximately 80%) and improved    degree of ordering, an increase in axial thermal conductivity of the    filament to >1,600 W/mK, and/or in-plane electrical conductivity to    5,000-7,000 S/cm.-   Regime 4: 2,000° C.-3,000° C. (the re-crystallization and perfection    regime); Oxygen content reduced to typically from near 0%-0.001%,    resulting in a reduction of inter-graphene spacing to approximately    0.3354 nm (degree of graphitization from 80% to nearly 100%) and    perfection of crystal structure and orientation, an increase in    axial thermal conductivity of the filament to >1,700 W/mK, and axial    electrical conductivity to 10,000-20,000 S/cm.

The degree of graphitization, g, was calculated from the X-raydiffraction pattern using Mering's Eq, d₀₀₂=0.3354 g+0.344 (1−g), whered₀₀₂ is the interlayer spacing of graphite or graphene crystal in nm.This equation is valid only when d₀₀₂ is no greater than 0.3440 nm. Theunitary graphene-based filament having a d₀₀₂ higher than 0.3440 nmreflects the presence of oxygen-containing functional groups (such as—OH, >O, and —COOH) and/or other chemical functional groups, such as—NH₂, on graphene molecular plane surfaces that act as a spacer toincrease the inter-graphene spacing.

Another structural index that can be used to characterize the degree ofordering of the presently invented unitary graphene filaments or relatedgraphite crystals is the “mosaic spread” value, which is expressed bythe full width at half maximum of the (002) or (004) reflection in aX-ray diffraction intensity curve. This degree of ordering characterizesthe graphite or graphene crystal size (or grain size), amounts of grainboundaries and other defects, and the degree of preferred grainorientation. A nearly perfect single crystal of graphite ischaracterized by having a mosaic spread value of 0.2-0.4. Most of ourunitary graphene materials (including filaments and films) have a mosaicspread value in this range of 0.2-0.4 (with a heat treatment temperatureno less than 2,000° C.). However, some values are in the range of0.4-0.7 if the ultimate heat treatment temperature (TTT) is between1,250 and 2,000° C., and in the range of 0.7-1.0 if the TTT is between600 and 1,250° C.

It may be noted that the unitary graphene filament can be made into aunitary graphene structure, including a graphene single crystal orpoly-crystal with few grain boundaries. This unitary graphene structurewould contain closely packed and bonded parallel graphene planes havingan inter-graphene plane spacing of 0.3354 to 0.40 nm (mostly between0.3354 and 0.337 nm) and an oxygen content up to 10% by weight (mostly<<1%). This unitary graphene structure can be obtained fromheat-treating a graphene oxide gel at a temperature higher than 100° C.(up to 600, 1,250, 2,000, or 3,000° C., depending upon the desiredproperties), wherein an average mis-orientation angle between twographene planes is less than 10 degrees, preferably and typically lessthan 5 degrees. The graphene single crystal refers to the single-grainor single-domain graphene or poly-crystalline structure (but havingincomplete grain boundaries) in which most of the graphene planes in allgrain(s) are essentially parallel to one another. They are all parallelto the fiber-axis direction. This unitary graphene structure or graphenemonolith contains therein no discrete graphite flake or grapheneplatelet derived from the graphene oxide gel. All graphene oxidemolecules have been chemically merged, linked, and integrated into onesingle integral unit, hence the name “unitary graphene” entity.

The unitary graphene filament typically and preferably has a physicaldensity of at least 1.7 g/cm³ or a porosity level lower than 10%, andmore typically and preferably has a physical density of at least 1.8g/cm³ or a porosity level lower than 5%. The process enables us toproduce unitary graphene fiber to reach a physical density mosttypically in the range of 1.9-2.0 g/cm³, approaching the theoreticaldensity of a perfect graphite single crystal. Yet, no conventionalgraphite single crystal can be readily produced to have a dimensionlarger than a few microns (μall). We can produce this giant graphenefilament or longer than tens of centimeters that are practically asingle crystal. This is most astonishing.

In an embodiment, the graphene oxide gel is obtained from a graphiticmaterial having a maximum original graphite grain size (L_(g)) and theunitary graphene material is a single crystal or a poly-crystal graphenestructure having a grain size larger than even the maximum originalgrain size. This maximum original grain size L_(g) is the largest lengthor width of a graphene plane or of a graphite crystallite in a graphiteparticle prior to being oxidized (L_(g)≥L_(a) and L_(g)≥L_(b), whereL_(a) and L_(b) are lateral dimensions of grains or graphene domains inthe original graphite particle, to be further defined later). The heattreatment involves extensive merging and linking of highly reactive GOmolecules to form huge graphene planes and huge graphene domains (orgrains) that are typically orders of magnitude greater than the originalgrain sizes.

The heat treatment, or chemical linking and re-graphitization treatment,thermally converts the GO molecules to an integrated graphene entity bychemically merging individual graphene oxide molecules primarily sidewayin an edge-to-edge manner to form significantly larger graphene planes,but sometimes also chemically linking with the GO molecules below orabove this graphene plane to form a 3-D molecular network. This 3-Dmolecular network can be broken and re-organized if the final heattreatment occurs at a sufficiently high temperature for an extendedlength of time.

The graphene oxide gel-derived unitary graphene-based graphitic fibersand fiber yarns have the following novel, unique, and unprecedentedcharacteristics:

-   -   (1) The unitary graphene filament is an integrated graphene        object that is either a graphene single crystal or a        poly-crystal having multiple grains (but with incomplete or        poorly delineated grain boundaries, or huge grain sizes, having        negligible amount of grain boundaries that would otherwise        impede flow of electrons and phonons). When made into a filament        under the influence of a shear stress (to induce viscosity        thinning associated with ordering of GO molecules), the unitary        graphene filament is composed of multiple graphene planes        essentially all of which are oriented parallel to one another        along the fiber axis direction.    -   (2) In contrast to the conventional spun graphene fibers, which        are porous aggregates of discrete graphene sheets twisted        together (e.g. those prepared by a spinning-coagulation or        constrained-length hydrothermal process), this integrated        graphene entity (the unitary graphene-based graphitic fiber) is        not an aggregate or stack of multiple discrete graphite flakes        or discrete sheets of graphene, GO, or RGO. This is a single        graphene entity or monolith. This unitary graphene entity does        not contain discrete graphite flakes or discrete graphene sheets        dispersed therein that are derived from the GO gel. The GO        molecules do not revert back to individual or discrete graphene        sheets or graphite flakes. Through chemical inter-linking of GO        molecules, re-graphitization, and re-crystallization, the GO        molecules and the original graphene planes of hexagonal carbon        atoms (that constitute the original graphite particles) have        completely lost their original individual identity and have been        united into one single entity (unitary body or monolith).    -   (3) The presently invented graphitic fiber is a neat graphene or        graphitic material without any binder, resin, matrix, or glue.        The integrated graphene entity is not made by gluing or bonding        discrete sheets/platelets together with a binder, linker, or        adhesive. Instead, GO molecules in the GO gel are merged, mainly        edge-to-edge through joining or forming of chemical bonds with        one another, into an integrated graphene entity, without using        any externally added linker or binder molecules or polymers.    -   (4) This unitary or monolithic graphene entity is derived from a        GO gel, which is in turn obtained from natural graphite or        artificial graphite particles originally having multiple        graphite crystallites. Prior to being chemically oxidized, these        starting graphite crystallites have an initial length (L_(a) in        the crystallographic a-axis direction), initial width (L_(b) in        the b-axis direction), and thickness (L_(c) in the c-axis        direction). The resulting unitary graphene entity typically has        a length or width significantly greater than the L_(a) and L_(b)        of the original crystallites. Even the individual grains in a        poly-crystalline unitary graphene entity have a length or width        significantly greater than the L_(a) and L_(b) of the        crystallites of original graphite particles (as the starting        material). They can be as large as the length or width of the        unitary graphene fiber itself, not just 2 or 3 times higher than        the initial L_(a) and L_(b) of the original crystallites. The        unitary graphene fiber has grain sizes typically no less than 10        μm, more typically no less than 100 μm, and even more typically        no less than 1 cm in the fiber axis direction.    -   (5) The mechanical stress-induced graphene molecular orientation        control, coupled with the nearly perfect graphene planes derived        from the well-aligned graphene molecules, enable us to achieve        both high strength and high Young's modulus with the presently        invented continuous graphitic fibers. This has not been possible        with conventional continuous carbon or graphite fibers. For        instance, ultra-high strength could only be obtained with        PAN-based carbon/graphite fibers, and ultra-high modulus could        only be obtained with pitch-based carbon/graphite fibers.    -   (6) The nearly perfect graphitic crystal structure with        essentially all constituent graphene planes being parallel to        the fiber axis direction has enabled the presently invented        graphitic fibers to exhibit tensile strength and Young's modulus        an order of magnitude higher than those of prior art graphene        fibers obtained via spinning-coagulation and hydrothermal        processes. Further, the electrical conductivity values of our        graphitic fibers are typically 2-3 orders of magnitude higher        (not just 2-3 times). The thermal conductivity has also reached        a value (e.g. 1,000-1,800 W/mK) that has never been obtained by        any continuous fibers.    -   (7) In summary, the continuous unitary graphene fibers, the        prior art continuous carbon/graphite fibers from PAN or pitch,        and prior art graphene fibers (e.g. prepared from the        coagulation route) are three fundamentally different and        patently distinct classes of materials in terms of chemical        composition, morphology, structure, process of production, and        various properties.        -   a. The presently invented graphitic fiber has a nearly            perfect graphitic crystal structure with essentially all            constituent graphene planes being parallel to each other and            parallel to the fiber axis direction. In addition, the            crystallographic c-axis directions of these graphene planes            are essentially pointing to the same direction, which does            not vary from point to point along the fiber axis direction.        -   b. In contrast, due to the chiral liquid crystalline nature            of the GO suspension used in prior art continuous graphene            fibers, these fibers are characterized by having many            separate strings of inter-connected graphene domains each            having a crystallographic c-axis. This crystallographic            c-axis of one domain is significantly different than the            crystallographic c-axis of the immediate adjacent graphene            domain, which is in turn different than that of the next            graphene domain along the same string. The crystallographic            c-axis follows a more or less helical pattern along a            particular string and the helical pitch (or period) of one            string is generally different than the pitch of an adjacent            string.        -   c. The presently invented graphitic fiber is essentially            pore-free with porosity level typically less than for 2% by            volume, but the prior art graphene fibers are inherently            very porous, typically having a porosity level in the range            of 10%-80% by volume.        -   d. The presently invented graphitic fiber has most of the            grain sizes being higher than 5 μm, mostly higher than 10            μm, often greater than 100 μm, with many in the centimeter            ranges. In contrast, the prior art graphene fibers have most            of the grain size or graphene domain size less than 2 μm,            mostly less than 1 μm. The PAN- and pitch-based            carbon/graphite fibers typically have the length of graphene            sheets less than 100 nm, mostly less than 30 nm.        -   e. The presently invented graphitic fiber is composed of            essentially 95-99.5% graphene planes with less than 5%            (mostly <1%) disordered structure or defects. In contrast,            the PAN- and pitch-based carbon/graphite fibers have a large            proportion of disordered and defected zones, typically much            higher than 5-10% in volume. Further, all the continuous            PAN- and pitch-based carbon/graphite fibers have a            core-shell structure with the shell being made up of a hard            carbon or amorphous carbon composition. In contrast, the            presently invented graphitic fibers do not have a shell or a            core-shell structure; all ingredients being graphene planes.        -   f. Some of the presently invented GO gel-derived fibers can            have a finite oxygen content (0.01 to 2% by weight) residing            externally, and an inter-graphene spacing of 0.34-0.40 nm            (dues to the presence of internal oxygen atoms) unless heat            treated at a temperature higher than approximately 1,500° C.            All the continuous PAN- and pitch-based carbon/graphite            fibers have an inter-graphene spacing less than 0.338 nm.

Another embodiment of the present invention is a process for producing acontinuous graphitic fiber from sheets or platelets of pristinegraphene, graphene oxide, or reduced graphene oxide. The processcomprises (a) preparing a graphene suspension having graphene sheetsdispersed in a fluid medium; (b) dispensing and depositing at least acontinuous filament of the graphene suspension onto a supportingsubstrate under the influence of a stress field to induce alignment orordering of graphene sheets along a filament axis direction; (c)partially or completely removing the fluid medium from the continuousfilament to form a continuous graphene fiber containing closely packedand parallel graphene sheets; and (d) heat treating the continuousgraphene fiber to form the desired continuous graphitic fiber at a heattreatment temperature higher than 600° C. to an extent that aninter-plane spacing d₀₀₂ is decreased to a value in the range from0.3354 nm to 0.4 nm. Since these sheets or platelets of pristinegraphene, graphene oxide, or reduced graphene oxide are already dead(not living molecules), the continuous graphene fiber would require amuch higher final heat treatment temperature to accomplishgraphitization and re-crystallization as compared with the fiber derivedfrom living GO molecules disclosed above. This graphitization andre-crystallization are only possible if the graphene fiber containsclosely packed and parallel graphene sheets. The requirement of graphenesheets being close-packed and parallel to one another could be met ifthe graphene suspension is dispensed and deposited under the influenceof a proper stress field. This requirement could not be met with theprior art continuous graphene fibers prepared by, for instance,spinning-coagulation.

Another embodiment of the present invention is a filter that contains apresently invented fabric as a filtering element. Such a filter is foundto have the best combination of filtering efficiency, filter usefullife, filter strength and structural integrity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) A flow chart illustrating various prior art processes ofproducing exfoliated graphite products (flexible graphite foils andflexible graphite composites), along with a process for producinggraphene oxide gel 21, oriented GO filament 35, and unitarygraphene-based fiber 37;

FIG. 1(b) Schematic drawing illustrating the processes for producingconventional paper, mat, film, and membrane of simply aggregatedgraphite or graphene flakes/platelets. All processes begin withintercalation and/or oxidation treatment of graphitic materials (e.g.natural graphite particles);

FIG. 1(c) Two types of fiber cross-sections (circular and rectangular)forming two types of yarns with different packing factors.

FIG. 2(a) A SEM image of a graphite worm sample after thermalexfoliation of graphite intercalation compounds (GICs) or graphite oxidepowders;

FIG. 2(b) An SEM image of a cross-section of a flexible graphite foil,showing many graphite flakes with orientations not parallel to theflexible graphite foil surface and also showing many defects, kinked orfolded flakes;

FIG. 2(c) SEM images of an elongated section of prior art graphenefibers produced by solution spinning and liquid coagulation, showingmany graphene sheets with orientations not parallel to the fiber axisdirection and also showing many defects, pores, kinked or foldedgraphene sheets.

FIG. 2(d) SEM images of an elongated section of prior art graphenefibers produced by solution spinning and liquid coagulation, showingmany graphene sheets with orientations not parallel to the fiber axisdirection and also showing many defects, pores, kinked or foldedgraphene sheets.

FIG. 3(a) A SEM image of a GO-derived graphene fiber. Original graphiteparticles having multiple graphene planes (with a length/width of 30nm-2 μm) were oxidized, exfoliated, re-oriented, and seamlessly mergedinto continuous-length graphene planes;

FIG. 3(b) A SEM image of a cross-section of a conventional graphenepaper/film prepared from discrete graphene sheets/platelets using apaper-making process (e.g. vacuum-assisted filtration). The image showsmany discrete graphene sheets being folded or interrupted (notintegrated), with orientations not parallel to the film/paper surfaceand having many defects or imperfections;

FIG. 3(c) Schematic drawing to illustrate the formation process of aunitary graphene fiber composed of multiple graphene planes that areparallel to one another and are well-bonded in the thickness-directionor crystallographic c-axis direction;

FIG. 3(d) One plausible chemical linking mechanism (only 2 GO moleculesare shown as an example; a large number of GO molecules can bechemically linked together to form a unitary graphene fiber).

FIG. 4(a) Schematic diagram illustrating a process of producing multiplecontinuous graphitic fibers from living GO molecules dispensed throughmultiple nozzles under the influence of shear stresses or strains.

FIG. 4(b) Schematic diagram illustrating a process of producing multiplecontinuous graphitic fibers from living GO molecules dispensed throughmultiple nozzles under the influence of shear stresses or strains.

FIG. 5(a) Thermal conductivity values of the GO gel-derived unitarygraphene-based continuous fibers and the fibers produced by spinning ofGO suspension into a coagulation bath, plotted as a function of thefinal heat treatment temperature. Conductivity values from twohigh-conductivity graphite fibers (K-1100 and P2 from Amoco) areincluded for comparison purpose.

FIG. 5(b) Electrical conductivity values of the GO gel-derived unitarygraphene-based continuous fibers and the fibers produced by spinning ofGO suspension into a coagulation bath, both plotted as a function of thefinal heat treatment temperature. Conductivity values from twohigh-conductivity graphite fibers (K-1100 and P2 from Amoco) areincluded for comparison purpose.

FIG. 6(a) Tensile strength of the GO gel-derived unitary graphene-basedcontinuous fibers plotted as a function of the final heat treatmenttemperature.

FIG. 6 (b) Young's modulus of the GO gel-derived unitary graphene-basedcontinuous fibers plotted as a function of the final heat treatmenttemperature;

FIG. 6(c) Tensile strength is plotted as a function of the Young'smodulus of the same fibers.

FIG. 7(a) X-ray diffraction curves of a GO filament (dried GO gelfilament),

FIG. 7(b) X-ray diffraction curves of GO filament thermally reduced at150° C. (partially reduced),

FIG. 7(c) X-ray diffraction curves of highly reduced and re-graphitizedunitary graphene filament, and

FIG. 7(d) X-ray diffraction curves of highly re-graphitized andre-crystallized graphitic fiber (a more advanced unitary graphenematerial) showing a high-intensity (004) peak.

FIG. 8(a) Inter-graphene plane spacing measured by X-ray diffraction;FIG. 8 (b) the oxygen content in the GO gel-derived unitary graphenefilaments; and FIG. 8 (c) correlation between inter-graphene spacing andthe oxygen content.

FIG. 9(a) linear-linear scale viscosity values of graphene gel plottedas a function of viscometer spindle speed (proportional to a shearrate).

FIG. 9(b) log-linear scale viscosity values of graphene gel plotted as afunction of viscometer spindle speed (proportional to a shear rate).

FIG. 9(c) log-log scale viscosity values of graphene gel plotted as afunction of viscometer spindle speed (proportional to a shear rate).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a fabric comprising a first layer ofyarns woven to form the fabric wherein the yarns comprise at least oneunitary graphene-based continuous graphitic fiber comprising at least90% by weight of graphene planes that are chemically bonded with oneanother having an inter-planar spacing d₀₀₂ from 0.3354 nm to 0.4 nm asdetermined by X-ray diffraction and an oxygen content less than 5% byweight (typically from 0.001% to 5% by weight), wherein the grapheneplanes are parallel to one another and parallel to a fiber axisdirection and the graphitic fiber contains no core-shell structure, hasno helically arranged graphene domains or domain boundary, and has aporosity level less than 10% by volume (more typically <5%). In apreferred embodiment, the inter-plane spacing d₀₀₂ is from 0.3354 nm to0.36 nm, the oxygen content is less than 2% by weight, and/or porositylevel is less than 2% by volume.

One unique and technologically significant characteristic of this fabricis that the constituent fibers derived from living graphene chains canbe made into a more or less rectangular cross-section (e.g. asschematically shown in FIG. 1(c)). As a consequence, the yarnscontaining multiple continuous fibers can have a cross-section that isrectangular or flat-shaped. A flat-shaped fiber or yarn has across-section with a width-to-thickness ratio of at least 2, preferablyat least 3, more preferably at least 5, but can be from 1.5 to 1,000.When one combines multiple filaments together (e.g. of thoseconventional fibers with a circular cross-section or irregular-shapecross-section), there is a limit to the packing factor. The highestpacking factor is typically between 50% and 65% by volume even forcircular-cross-section fibers. In contrast, as shown in FIG. 1(c), thepresently invented rectangular or flat-shaped graphene fibers can bepacked into a yarn with an essentially 100% packing factor, if sodesired. The packing factor can be adjusted to be between 20% and 100%,preferably between 40% and 95%, more preferably between 60% and 90%, andmost preferably between 70% and 85% for composite structure or filterapplications. Our research data have demonstrated that the flexuralstrength and elastic modulus values of polymer matrix compositescontaining presently invented graphitic fiber-based fabrics as areinforcement phase are significantly higher than those of thecomposites containing a comparable volume fraction of conventionalgraphitic fibers. The differences are typically between 30% and 300%.

As another example, fabrics that block entry of pathogenic agentsrequire that the yarns of highest packing factors be interwoven tightlyto prevent any gaps between one another. The thickness and shapes ofconventional fibers alone could allow significant gaps within each yarndefined by those fibers. The instant invention provides tightly packedyarns and fabrics. These features are not achievable with conventionalgraphitic fibers.

The fabric, the yarns, or the continuous graphitic fiber can have across-section that is rectangular or flat-shaped, having a width and athickness. Preferably, the fabric, the yarns, or the graphitic fibershave a width-to-thickness ratio greater than 5, more preferably >20, andcan be greater than 150. The fabric or the yarn can have a thicknessless than 1 μm, or even less than 100 nm. Conventional continuousgraphitic fiber yarns cannot be made into a fabric having a thicknessless than 1 μm or less than 100 nm.

One embodiment of the present invention is a continuous graphiticfiber-based yarn and fabric produced from living graphene molecules. Theyarn is composed of multiple continuous fibers wherein at least one ofthe fibers is a graphene-based graphitic fiber produced by a processpreferably comprising: (a) preparing a graphene oxide gel having livinggraphene oxide molecules or functionalized graphene chains dissolved ina fluid medium wherein the graphene oxide molecules contain an oxygencontent higher than 10% by weight (preferably higher than 20% byweight); (b) dispensing and depositing at least a continuous filament ofgraphene oxide gel onto a supporting solid substrate, wherein thedispensing and depositing procedure includes mechanical stress-inducedmolecular alignment of the living graphene oxide molecules orfunctionalized graphene chains along a filament axis direction; (c)partially or completely removing the fluid medium from said continuousfilament to form a continuous graphene oxide fiber, wherein saidgraphene oxide fiber has an inter-plane spacing d₀₀₂ of 0.4 nm to 1.2 nmas determined by X-ray diffraction and an oxygen content no less than10% by weight; and (d) heat treating the continuous graphene oxide fiberto form the continuous graphitic fiber at a heat treatment temperaturehigher than 100° C. (preferably >600° C. and more preferably >1,000° C.)to the extent that an inter-plane spacing d₀₀₂ is decreased to a valuein the range of 0.3354 nm to 0.4 nm and the oxygen content is decreasedto less than 5% by weight (preferably less than 2%). Preferably,multiple continuous graphitic fibers of this type are then formed intoyarns of a desired shape. Multiple yarns of this type of continuousgraphitic fibers, alone or in combinations with other types of fibers oryarns, are made into a fabric.

Since step (b) involves dispensing and depositing GO gel onto a solidsubstrate, this process has essentially excluded anyspinning-coagulation process that involves spinning liquid crystallineGO solution into a coagulation bath containing a liquid non-solvent.Coagulation inherently randomizes the orientation of graphene sheets,which is in contrast to our intent to achieve preferred orientations ofthe graphene planes of carbon atoms along the fiber axis.

It is important to note that multiple filaments can be producedconcurrently if we dispense and form multiple continuous filaments of GOgel onto a supporting substrate. There is no limitation as to how manyfilaments can be generated at the same time. Hundreds, thousands, ortens of thousands of filaments can be made and combined into acontinuous yarn when or after these filaments are made. Preferably,however, these filaments are not combined to form a yarn until thefilaments have been heat-treated to become graphitic fibers with desiredfiber characteristics. The yarn can contain from just one GO gel-derivedgraphitic fiber (the rest being other types of fibers) to all fibersbeing derived from GO gel, depending upon the desired yarn properties.

In a more preferred embodiment, step (c) includes forming a grapheneoxide filament having an inter-plane spacing d₀₀₂ of 0.4 nm to 0.7 nmand an oxygen content no less than 20% by weight; and step (d) includesheat-treating the graphene oxide filament to an extent that aninter-plane spacing d₀₀₂ is decreased to a value in the range of 0.3354nm to 0.36 nm and the oxygen content is decreased to less than 2% byweight (most preferably between 0.001% to 0.01% by weight).

The unitary graphene-based graphitic fiber is obtained fromheat-treating a graphene oxide gel filament at a temperature higher than100° C. (preferably higher than 600° C., more preferably higher than1,250° C., further preferably higher than 2,000° C., and advantageouslyhigher than 2,500° C. if a perfect or nearly perfect graphene crystal isdesired) and contains chemically bonded graphene molecules. These planararomatic molecules or graphene planes (hexagonal structured carbonatoms) are parallel to one another. The lengths of the un-interruptedplanes along the fiber axis are huge, typically several times or evenorders of magnitude larger than the maximum crystallite dimension (ormaximum constituent graphene plane dimension) of the starting graphiteparticles. The presently invented unitary graphene filament is a “giantgraphene crystal” or “giant graphene fibers” having essentially allconstituent graphene planes being parallel to one another along thefiber axis. This is a unique and new class of material that has not beenpreviously discovered, developed, or suggested to possibly exist.

The graphene oxide gel is a very unique and novel class of material thatsurprisingly has great cohesion power (self-bonding, self-polymerizing,and self-crosslinking capability) and adhesive power (capable ofchemically bonding to a wide variety of solid surfaces). Thesecharacteristics have not been taught or hinted in the prior art. The GOgel is obtained by immersing powders of a starting graphitic material inan oxidizing liquid medium (e.g. a mixture of sulfuric acid, nitricacid, and potassium permanganate) in a reaction vessel. The startinggraphitic material may be selected from natural graphite, artificialgraphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead,soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbonnano-tube, or a combination thereof.

When the starting graphite powders are dispersed in the oxidizing liquidmedium, the resulting slurry (heterogeneous suspension) initiallyappears completely dark and opaque. When the oxidation of graphiteproceeds at a reaction temperature for a sufficient length of time undera controlled pH condition, the reacting mass can eventually become ahomogeneous solution having no discernible or visually identifiabledispersed solid particle (as opposed to the initially heterogeneoussuspension that contain identifiable solid particles). The solution canbe optically translucent or transparent or brown-colored, which alsolooks and behaves like a polymer gel. This heavy oxidation-inducedgraphene oxide gel is composed of graphene oxide molecules dissolved inthe liquid medium. The graphene oxide molecules, prior to any subsequentheat treatment, have an oxygen content typically no less than 10-20% byweight (typically from 40-50% by weight) and their molecular weights aretypically less than 43,000 g/mole (often less than 4,000 g/mole, buttypically greater than 200 g/mole) while in a gel state. The grapheneoxide gel is composed of graphene oxide molecules dissolved in an acidicmedium having a pH value of typically no higher than 5.

The graphene oxide gel has a typical viscosity from 500 centipoise (cP)to 500,000 cP when measured at 20° C. prior to shear-induced thinning.The viscosity is more typically greater than 2,000 cP and less than300,000 cP when measured at 20° C. prior to the shear-induced thinningprocedure. Preferably, the viscosity of the GO gel as a precursor tounitary graphene material is in the range of 2,000-50,000 cP.Preferably, the GO gel is subjected to a shear stress field that theviscosity is reduced to below 2,000 cP (or even below 1,000 cP) duringor after shear-induced thinning. In an embodiment, the graphene oxidegel has a viscosity greater than 5,000 cP when measured at 20° C. priorto shear-induced thinning, but is reduced to below 5,000 cP (preferablyand typically below 2,000 cP or even below 1,000 cP) during or aftershear-induced thinning. The viscosity data measured at 20° C., shown inFIG. 9(a), FIG. 9(b), and FIG. 9(c) as an example, clearly indicate thateven an ultra-high viscosity value (e.g., 300,000 cP) can be reduceddown to 1,000-2,000 cP with a sufficiently high shear rate. This is areduction by more than 2 orders of magnitude, a highly unexpectedobservation. The straight line of the data when plotted in a log-logscale indicates a shear thinning fluid flow behavior.

In step (b), the GO gel is formed into a filamentary shape preferablyunder the influence of a shear stress or strain. One example of such ashearing procedure is casting or coating a thin string of GO gel(gel-like fluid) using a dispensing or extrusion machine. This procedureis similar to coating a very narrow-width strip or filament of varnish,paint, or coating onto a solid substrate. The roller, “doctor's blade”,or wiper creates a shear stress when the thin filament is being shaped,or when a relative motion is conducted between the roller/blade/wiperand the supporting substrate. Quite unexpectedly and significantly, sucha shearing action reduces the effective viscosity of the GO gel andenables the planar graphene oxide molecules to well align along, forinstance, the shearing direction or the filament axis direction. Furthersurprisingly, such a molecular alignment state or preferred orientationis not disrupted when the liquid components in the GO gel aresubsequently removed to form a well-packed GO filament that is at leastpartially dried. The dried GO filament has a high birefringencecoefficient between the axial direction and the transverse direction.

This mechanical stress/strain also enables all the constituent grains orgraphene domains along a graphitic fiber remain substantially parallelto one another. In other words, not only the graphene planes in aparticular domain are parallel to one another, they are also parallel tothe graphene planes in the adjacent domain. The crystallographic c-axesof these two sets of graphene planes are pointing along substantiallyidentical direction. In other words, the domains or grains do not followa helical or twisting pattern. Thus, the continuous graphitic fibercontains a first graphene domain containing bonded graphene planesparallel to one another and having a first crystallographic c-axis, anda second graphene domain containing bonded graphene planes parallel toone another and having a second crystallographic c-axis wherein thefirst crystallographic c-axis and the second crystallographic c-axis areinclined with respect to each other at an angle less than 10 degrees(mostly less than 5% and even more often less than 1 degree).

As schematically illustrated in FIG. 4(a) and FIG. 4(b), multipledispensing devices or one dispensing device with multiple nozzles may beused to dispense multiple filaments of GO gel onto a moving substrate ina continuous manner. A feeder roller provides a solid substrate (e.g.plastic film) that moves from the left side to the right side of FIG.4(a) and is collected on a take-up roller. A drying/heating zone may beimplemented to remove most of the liquid component (e.g. water) from theGO gel filaments prior to being collected on the winding roller.Multiple filaments of GO gel may be laid onto the substrate. Thisdeposition step should preferably involve a local shear stress/strainexerted on the GO gel filaments for the purpose of assembling the planarGO molecules into an ordered and aligned structure.

For instance, the relative movement between the substrate (carrying theGO gel filament mass) and a blade/wiper may be sufficient to force theplanar GO molecules to align themselves along the filament axisdirection (or the substrate moving direction). The planar GO moleculesare self-assembled to be parallel to the substrate surface plane in anordered and overlaying manner. Such an ordered packing or self-assembledconfiguration unexpectedly proves to be conducive to subsequentheat-induced chemical linking and merging between GO molecules andfurther re-graphitization and re-crystallization of graphitic domains.This is in stark contrast to the coagulation procedure in the prior artgraphene fiber production process, wherein coagulation inherentlyprecipitates out isolated GO sheets that are separated from one another,eliminating the possibility of packing and aligning these GO sheets foreffective chemical linking and re-graphitization.

This dried GO filament is then subjected to a properly programmed heattreatment that can be divided into four distinct heat treatmenttemperature (HTT) regimes:

-   Regime 1 (approximately 100° C.-600° C.): In this temperature range    (the thermal reduction regime), the GO filament primarily undergoes    thermally-induced reduction reactions, leading to a reduction of    oxygen content from typically 30-50% (as dried) to 5-6%. This    treatment also results in a reduction of inter-graphene spacing from    approximately 0.6-1.0 nm (as dried) to approximately 0.4 nm and an    increase in axial thermal conductivity from approximately 50-100    W/mK to 450 W/mK. Even with such a low temperature range, some    chemical linking occurs. The GO molecules remain well-aligned, but    the inter-GO spacing remains relatively large (0.4 nm or larger).    Many O-containing functional groups and other functional groups    survive.-   Regime 2 (approximately 600° C.-1,250° C.): In this chemical linking    regime, extensive chemical combination, polymerization (combination    of GO chains), and cross-linking between adjacent GO or    functionalized molecules occur. The oxygen content is reduced to    typically 0.7% (<<1%), resulting in a reduction of inter-graphene    spacing to approximately 0.345 nm. This implies that some initial    graphitization (or re-graphitization) has already begun at such a    low temperature, in stark contrast to conventional graphitizable    materials (such as carbonized PAN fiber) that typically require a    temperature as high as 2,500° C. to initiate effective    graphitization. This is another distinct feature of the presently    invented unitary graphene-based graphitic fibers and its production    processes. These chemical linking reactions result in an increase in    axial thermal conductivity of a unitary graphene-based fiber to    1,000-1,200 W/mK, and/or axial electrical conductivity to the range    of 3,000-5,000 S/cm.-   Regime 3 (approximately 1,250° C.-2,000° C.): In this ordering and    re-graphitization regime, extensive graphitization or graphene plane    merging occurs, leading to significantly improved degree of    structural ordering. As a result, the oxygen content is reduced to    typically 0.01% and the inter-graphene spacing to approximately    0.337 nm (achieving degree of graphitization from 1% to    approximately 80%, depending upon the actual HTT and length of    time). The improved degree of ordering is also reflected by an    increase in axial thermal conductivity to >1,600 W/mK, and/or axial    electrical conductivity to 5,000-8,000 S/cm.-   Regime 4 (approximately 2,000° C.-3,000° C. or higher): In this    re-crystallization and perfection regime, extensive movement and    elimination of grain boundaries and other defects occur, resulting    in the formation of perfect or nearly perfect single crystals, or    poly-crystalline graphene crystals with incomplete grain boundaries    or huge grains (these grains can be orders of magnitude larger than    the original grain sizes of the starting graphite particles for GO    gel production. The oxygen content is essentially eliminated,    typically 0%-0.001%. The inter-graphene spacing is reduced to down    to approximately 0.3354 nm (degree of graphitization from 80% to    nearly 100%), corresponding to that of a perfect graphite single    crystal. Quite interestingly, the graphene single crystal or    poly-crystal has all the graphene planes being closely packed and    bonded, and all aligned along one direction, a perfect orientation.    Such a perfectly oriented structure has not been produced even with    the highly oriented pyrolytic graphite (HOPG) being subjected    concurrently to an ultra-high temperature (3,400° C.) under an    ultra-high pressure (300 Kg/cm²). The unitary graphene-based    continuous fiber can achieve such a highest degree of perfection    with a significantly lower temperature and an ambient (or slightly    higher compression) pressure. The unitary graphene-based graphitic    fiber thus obtained exhibits an axial thermal conductivity up    to >1,800 W/mK, and electrical conductivity to 10,000-20,000 S/cm.    No continuous fiber of any type thus far has exhibited these    exceptional conductivity values.

The presently invented unitary graphene material can be obtained byheat-treating the dried GO mass with a temperature program that coversat least the first regime (typically requiring 1-4 hours in thistemperature range if the temperature never exceeds 600° C.), morecommonly covers the first two regimes (1-2 hours preferred), still morecommonly the first three regimes (preferably 0.5-2.0 hours in Regime 3),and most commonly all the 4 regimes (Regime 4, for 0.2 to 1 hour, may beimplemented to achieve the highest conductivity and Young's modulus).

X-ray diffraction patterns were obtained with an X-ray diffractometerequipped with CuKcv radiation. The shift and broadening of diffractionpeaks were calibrated using a silicon powder standard. The degree ofgraphitization, g, was calculated from the X-ray pattern using theMering's Eq, d₀₀₂=0.3354 g+0.344 (1−g), where d₀₀₂ is the interlayerspacing of graphite or graphene crystal in nm. This equation is validonly when d₀₀₂ is equal or less than approximately 0.3440 nm. Theunitary graphene material or lightly oxidized graphite crystallinematerial having a d₀₀₂ higher than 0.3440 nm reflects the presence ofoxygen-containing functional groups (such as —OH, >O, and —COOH ongraphene molecular plane surfaces) that act as a spacer to increase theinter-graphene spacing. Conventional continuous carbon/graphite fibersdo not have these oxygen-containing groups residing in the interior ofthe fiber.

Another structural index that can be used to characterize the degree ofordering of the presently invented unitary graphene material andconventional graphite crystals is the “mosaic spread,” which isexpressed by the full width at half maximum of a rocking curve (X-raydiffraction intensity) of the (002) or (004) reflection. This degree ofordering characterizes the graphite or graphene crystal size (or grainsize), amounts of grain boundaries and other defects, and the degree ofpreferred grain orientation. A nearly perfect single crystal of graphiteis characterized by having a mosaic spread value of 0.2-0.4. Most of ourunitary graphene materials have a mosaic spread value in this range of0.2-0.4 (with a heat treatment temperature no less than 2,000° C.).However, some values are in the range of 0.4-0.7 if the highest heattreatment temperature (TTT) is between 1,250° C. and 2,000° C., and inthe range of 0.7-1.0 if the TTT is between 600 and 1,250° C.

The heat treatment temperature conditions for GO are such that theunitary graphene-based fiber is relatively pore-free having a physicaldensity of at least 1.6 g/cm³ or a porosity level lower than 10%. Undermore typical processing conditions, the unitary graphene-based graphiticfiber has a physical density of at least 1.7 g/cm³ or a porosity levellower than 5%. In most cases, the fiber has a physical density greaterthan 1.8 g/cm³ or a porosity level less than 2%. The chemically bondedgraphene planes in the unitary graphene oxide fiber typically contain acombination of sp² and sp^(a) electronic configurations (particularlyfor those unitary graphene materials prepared with the maximum treatmenttemperature lower than 2,000° C.). Above such a high temperature, mostof the bonding in the graphene planes is sp² and the bonding betweengraphene planes is van der Waals forces.

The graphene oxide (GO) gel-derived unitary graphene-based graphiticfibers and related processes have the following characteristics andadvantages:

-   (1) The unitary graphene-based fiber is an integrated graphene phase    that is either a graphene single crystal or a poly-crystal having    multiple grains with exceptionally large grains or incomplete grain    boundaries. When made into a filament under a desired shearing    stress field condition, the fiber is composed of very long,    chemically bonded graphene planes that are essentially oriented    parallel to one another. The grains in a graphene poly-crystal have    poorly delineated or incomplete grain boundaries. These grains are    essentially a single grain with some residual demarcation lines.    Such type of graphene poly-crystal is best described as a graphene    single crystal with some aligned but sporadic defects. These defects    can be eliminated to form a practically perfect single crystal if    the unitary graphene structure is allowed to undergo    re-crystallization at a temperature higher than approximately    2,500° C. for a sufficient length of time. This conclusion was drawn    after an extensive investigation using a combination of SEM, TEM,    selected area diffraction (with a TEM), X-ray diffraction, atomic    force microscopy (AFM), Raman spectroscopy, and FTIR.-   (2) The yarn-like graphene fibers prepared by the prior art    processes (e.g. spinning-coagulation) are a simple, un-bonded    aggregate/stack of multiple discrete platelets or sheets of    graphene, GO, or RGO that are mechanically fastened together. In    contrast, the unitary graphene fiber of the present invention is a    fully integrated, single graphene entity or monolith containing no    discrete sheets or platelets derived from the GO gel. All the GO    planes are covalently bonded along the fiber axis direction and    bonded at least with van der Waals forces in a transverse direction    (perpendicular to the fiber axis).-   (3) With these conventional processes, the constituent graphene    sheets of the resulting yarn-like fibers remain as discrete    flakes/sheets/platelets that can be easily discerned or clearly    observed. In a cross-sectional view under a SEM (e.g. FIG. 2(c)),    these discrete sheets are relatively random in orientation and have    many pores between these discrete sheets.

In contrast, the preparation of the presently invented unitary graphenefiber structure involves heavily oxidizing the original graphiteparticles, to the extent that practically every one of the originalgraphene planes has been oxidized and isolated from one another tobecome individual molecules that possess highly reactive functionalgroups (e.g. —OH, >O, and —COOH) at the edge and, mostly, on grapheneplanes as well. These individual hydrocarbon molecules (containingelements such as O and H, in addition to carbon atoms) are dissolved inthe reaction medium (e.g. mixture of water and acids) to form a gel-likemass, herein referred to as the GO gel. This gel is then dispensed andformed into a thin continuous filament onto a solid substrate surfaceunder shear stress field conditions. The liquid components are thenremoved to form a dried GO filament. When heated, these highly reactivemolecules react and chemically join with one another mostly in lateraldirections along graphene planes (in an edge-to-edge manner) and, insome cases, between graphene planes as well.

Illustrated in FIG. 3(d) is a plausible chemical linking mechanism whereonly 2 aligned GO molecules are shown as an example, although a largenumber of GO molecules can be chemically linked together to form aunitary graphene layer. Further, chemical linking could also occurface-to-face, not just edge-to-edge. These linking and merging reactionsproceed in such a manner that the molecules are chemically merged,linked, and integrated into one single entity or monolith. The moleculescompletely lose their own original identity and they no longer arediscrete sheets/platelets/flakes. There is only one single layer-likestructure (unitary graphene entity) that is one huge molecule or just anetwork of interconnected giant molecules with an essentially infinitemolecular weight. This may also be described as a graphene singlecrystal (with only one grain in the entire structure or entity, or apoly-crystal (with several large-sized grains, but typically nodiscernible, well-defined grain boundaries). All the constituentgraphene planes are very large in lateral dimensions (length and width)and, if produced under shear stress conditions and heat-treated at ahigher temperature (e.g. >1,250° C. or much higher), these grapheneplanes are essentially parallel to one another.

In-depth studies using a combination of SEM, TEM, selected areadiffraction, X-ray diffraction, AFM, Raman spectroscopy, and FTIRindicate that the graphene monolith is composed of several huge grapheneplanes (with length typically >>100 μm, more typically >>1 mm, andoften >>1 cm). These giant graphene planes are stacked and bonded alongthe thickness direction (crystallographic c-axis direction) oftenthrough not just the van der Waals forces (as in conventional graphitecrystallites), but also covalent bonds (if the ultimate heat treatmenttemperature is lower than 1,500-2,000° C.). Not to be limited by theory,but Raman and FTIR spectroscopy studies appear to indicate theco-existence of sp² (dominating) and sp³ (weak but existing) electronicconfigurations in these GO-derived fibers treated at lower temperatures,not just the conventional sp² in graphite.

-   (4) This integrated graphene entity is not made by gluing or bonding    discrete flakes/platelets together with a resin binder, linker, or    adhesive. Instead, GO molecules in the GO gel are merged through    joining or forming of covalent bonds with one another, into an    integrated graphene entity, without using any externally added    linker or binder molecules or polymers. Hence, the graphitic fiber    of the present invention is a neat graphene structure, containing    and involving no binder, no adhesive, and no matrix material.-   (5) This unitary or monolithic graphene entity typically has the    crystallographic c-axis in all grains being essentially parallel to    each other. This entity is derived from a GO gel, which is in turn    obtained from natural graphite or artificial graphite particles    originally having multiple graphite crystallites. Prior to being    chemically oxidized, these starting graphite crystallites have an    initial length (L_(a) in the crystallographic a-axis direction),    initial width (L_(b) in the b-axis direction), and thickness (L_(c)    in the c-axis direction). Upon heavy oxidation, these initially    discrete graphite particles are chemically transformed into highly    aromatic graphene oxide molecules having a significant concentration    of edge- or surface-borne functional groups (e.g. —OH, >C═O, and    —COOH, etc.). These aromatic GO molecules in the GO gel have lost    their original identity of being part of a graphite particle or    flake. Upon removal of the liquid component from the GO gel, the    resulting GO molecules are stacked upon one another in a relatively    ordered manner if the GO gel was under the influence of shear    stresses during or after dispensing/depositing operation. Upon heat    treatment, these GO molecules are chemically merged and linked into    a unitary or monolithic graphene entity that is highly ordered,    essentially a single crystal or poly-crystal with huge grains when    the temperature is sufficiently high.

The resulting unitary graphene entity typically has a lengthsignificantly greater than the L_(a) and L_(b) of the originalcrystallites. The grain size (length) of this unitary graphene-basedfiber is typically greater than the L_(a) and L_(b) of the originalcrystallites. They can be several orders of magnitude (not just 2 or 3times) higher than the initial L_(a) and L_(b) of the original graphitecrystallites.

-   (6) Due to these unique chemical compositions (including oxygen    content), morphology, crystal structure (including inter-graphene    spacing), and microstructural features (e.g. defects, incomplete or    lack of grain boundaries, chemical bonding and no gap between    graphene sheets, and no interruptions in graphene planes), the    graphene oxide gel-derived unitary or monolithic graphene-based    fiber has a unique combination of outstanding thermal conductivity,    electrical conductivity, tensile strength, and Young's modulus. No    prior art continuous fiber of any material type even comes close to    these combined properties.

Such graphitic fibers are expected to find application in formingcomposites for use where good dissipation of electrical charges or heatis desired. In addition, the combination of high stiffness and goodthermal conductivity with the near zero coefficient of thermal expansioncharacteristically exhibited by these graphene-derived graphitic fibersprovides composites that are of extraordinary dimensional stability.

Further, graphitic fibers will be widely used in the manufacture ofaircraft parts, space devices, precision machines, transportationvehicle components, sporting goods, and the like due to their excellentmechanical properties, such as specific strength, specific modulus, andchemical resistance. In such applications, the graphitic fiber isnormally used as reinforcement in composite materials comprising amatrix component such as a metal, graphitic carbon, a ceramic, a glass,a polymer, or the like. Graphitic fiber-reinforced composites havingsynthetic resins as a matrix are expected to find a broad array ofapplications in view of the combination of exceptional physical andchemical properties.

Fabricating composites is generally accomplished by processes such asfilament winding, pultrusion, and by layup and impregnation using tapeand fabric woven from fiber yarns. Thus, fiber yarns are considered asthe building blocks of many structural or functional composite products.

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 1(b), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene planes or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by van der Waals forces and groups of these graphene layersare arranged in crystallites that are typically oriented in differentdirections. The graphite crystallite structure is usually characterizedin terms of two axes or directions: the c-axis direction and the a-axis(or b-axis) direction. The c-axis is the direction perpendicular to thebasal planes. The a- or b-axes are the directions parallel to the basalplanes (perpendicular to the c-axis direction).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L_(c) along the crystallographic c-axisdirection. In particles of natural graphite, L_(a) and L_(b) aretypically in the range of 0.5 μm-100 μm and the L_(c) is typically lessthan 500 nm and often less than 100 nm. The constituent graphene planesof a crystallite are highly aligned or oriented with respect to eachother and, hence, these anisotropic structures give rise to manyproperties that are highly directional. For instance, the thermal andelectrical conductivity of a crystallite are of great magnitude alongthe plane directions (a- or b-axis directions), but relatively low inthe perpendicular direction (c-axis). As illustrated in the upper-leftportion of FIG. 1(b), different crystallites in a graphite particle aretypically oriented in different directions and, hence, a particularproperty of a multi-crystallite graphite particle is the directionalaverage value of all the constituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. In a typical process, particles of naturalgraphite (e.g. 100 in FIG. 1(b)) are intercalated in an acid solution toproduce graphite intercalation compounds (GICs, 102). The GICs arewashed, dried, and then exfoliated by exposure to a high temperature fora short period of time. This causes the flakes to expand or exfoliate inthe c-axis direction of the graphite up to 30-800 times of theiroriginal dimensions. The exfoliated graphite flakes are vermiform inappearance and, hence, are commonly referred to as graphite worms 104.These worms of graphite flakes which have been greatly expanded can beformed without the use of a binder into cohesive or integrated sheets ofexpanded graphite, e.g. webs, papers, strips, tapes, foils, mats or thelike (typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

The upper left portion of FIG. 1(a) shows a flow chart that illustratesthe prior art processes used to fabricate flexible graphite foils andthe resin-impregnated flexible graphite composite. The processestypically begin with intercalating graphite particles 20 (e.g., naturalgraphite or synthetic graphite) with an intercalant (typically a strongacid or acid mixture) to obtain a graphite intercalation compound 22(GIC). After rinsing in water to remove excess acid, the GIC becomes“expandable graphite.” The GIC or expandable graphite is then exposed toa high temperature environment (e.g., in a tube furnace preset at atemperature in the range of 800-1,050° C.) for a short duration of time(typically from 15 seconds to 2 minutes). This thermal treatment allowsthe graphite to expand in its c-axis direction by a factor of 30 toseveral hundreds to obtain a worm-like vermicular structure 24 (graphiteworm), which contains exfoliated, but un-separated graphite flakes withlarge pores interposed between these interconnected flakes. An exampleof graphite worms is presented in FIG. 2(a).

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is re-compressed by using a roll-pressing technique to obtainflexible graphite foils (26 in FIG. 1(a) or 106 in FIG. 1(b)), which aretypically much thicker than 100 μm. An SEM image of a cross-section of aflexible graphite foil is presented in FIG. 2(b), which shows manygraphite flakes with orientations not parallel to the flexible graphitefoil surface and there are many defects and imperfections.

Largely due to these mis-orientations of graphite flakes and thepresence of defects, commercially available flexible graphite foilsnormally have an in-plane electrical conductivity of 1,000-3,000 S/cm,through-plane (thickness-direction or Z-direction) electricalconductivity of 15-30 S/cm, in-plane thermal conductivity of 140-300W/mK, and through-plane thermal conductivity of approximately 10-30W/mK. These defects and mis-orientations are also responsible for thelow mechanical strength (e.g. defects are potential stress concentrationsites where cracks are preferentially initiated). In another prior artprocess, the exfoliated graphite worm 24 may be impregnated with a resinand then compressed and cured to form a flexible graphite composite 28,which is normally of low strength as well. In addition, upon resinimpregnation, the electrical and thermal conductivity of the graphiteworms could be reduced by two orders of magnitude.

Alternatively, the exfoliated graphite may be subjected tohigh-intensity mechanical shearing/separation treatments using ahigh-intensity air jet mill, high-intensity ball mill, or ultrasonicdevice to produce separated nano graphene platelets 33 (NGPs) with allthe graphene platelets thinner than 100 nm, mostly thinner than 10 nm,and, in many cases, being single-layer graphene (also illustrated as 112in FIG. 1(b). An NGP is composed of a graphene sheet or a plurality ofgraphene sheets with each sheet being a two-dimensional, hexagonalstructure of carbon atoms.

Further alternatively, with a low-intensity shearing, graphite wormstend to be separated into the so-called expanded graphite flakes (108 inFIG. 1(b) having a thickness >100 nm. These flakes can be formed intographite paper or mat 106 using a paper- or mat-making process. Thisexpanded graphite paper or mat 106 is just a simple aggregate or stackof discrete flakes having defects, interruptions, and mis-orientationsbetween these discrete flakes.

For the purpose of defining the geometry and orientation of an NGP, theNGP is described as having a length (the largest dimension), a width(the second largest dimension), and a thickness. The thickness is thesmallest dimension, which is no greater than 100 nm, preferably smallerthan 10 nm in the present application. When the platelet isapproximately circular in shape, the length and width are referred to asdiameter. In the presently defined NGPs, both the length and width canbe smaller than 1 μm, but can be larger than 200 μm.

A mass of multiple NGPs (including discrete sheets/platelets ofsingle-layer and/or few-layer graphene, 33 in FIG. 1(a)) may be madeinto a graphene film/paper (34 in FIG. 1(a) or 114 in FIG. 1(b)) using afilm- or paper-making process. FIG. 3(b) shows a SEM image of across-section of a graphene paper/film prepared from discrete graphenesheets using a paper-making process. The image shows the presence ofmany discrete graphene sheets being folded or interrupted (notintegrated), most of platelet orientations being not parallel to thefilm/paper surface, the existence of many defects or imperfections.

The precursor to the unitary graphene layer is graphene oxide gel 21(FIG. 1(a)). This GO gel is obtained by immersing a graphitic material20 in a powder or fibrous form in a strong oxidizing liquid in areaction vessel to form a suspension or slurry, which initially isoptically opaque and dark. This optical opacity reflects the fact that,at the outset of the oxidizing reaction, the discrete graphite flakesand, at a later stage, the discrete graphene oxide flakes scatter and/orabsorb visible wavelengths, resulting in an opaque and generally darkfluid mass. If the reaction between graphite powder and the oxidizingagent is allowed to proceed at a sufficiently high reaction temperaturefor a sufficient length of time, this opaque suspension is transformedinto a brown-colored and typically translucent or transparent solution,which is now a homogeneous fluid called “graphene oxide gel” (21 in FIG.1(a)) that contains no discernible discrete graphite flakes or graphiteoxide platelets. If dispensed and deposited under a shear stress field,the GO gel undergoes viscosity reduction and molecular orientation toform “oriented GO” 35, which can be heat-treated to become a unitarygraphene material 37.

Again, this graphene oxide gel is typically optically transparent ortranslucent and visually homogeneous with no discernible discreteflakes/platelets of graphite, graphene, or graphene oxide dispersedtherein. In the GO gel, the GO molecules are uniformly dissolved in anacidic liquid medium. In contrast, conventional suspension of discretegraphene sheets, graphene oxide sheets, and expanded graphite flakes ina fluid (e.g. water, organic acid or solvent) look dark, black or heavybrown in color with individual graphene or graphene oxide sheets orexpanded graphite flakes discernible or recognizable even with nakedeyes or a low-magnification light microscope (100×-1,000×).

The graphene oxide molecules dissolved in the liquid medium of agraphene oxide gel are aromatic chains that have an average number ofbenzene rings in the chain typically less than 1,000, more typicallyless than 500, and many less than 100. Most of the molecules have morethan 5 or 6 benzene rings (mostly >10 benzene rings) from combinedatomic force microscopy, high-resolution TEM, and molecular weightmeasurements. Based on our elemental analysis, these benzene-ring typeof aromatic molecules are heavily oxidized, containing a highconcentration of functional groups, such as —COOH and —OH and,therefore, are “soluble” (not just dispersible) in polar solvents, suchas water. The estimated molecular weight of these graphene oxidepolymers in the gel state is typically between 200 g/mole and 43,000g/mole, more typically between 400 g/mole and 21,500 g/mole, and mosttypically between 400 g/mole and 4,000 g/mole.

These soluble molecules behave like polymers and are surprisinglycapable of reacting and getting chemically connected with one another(during the subsequent heat treatment or re-graphitization treatment) toform a unitary graphene fiber of good structural integrity and highthermal conductivity. Conventional discrete graphene sheets, grapheneoxide sheets, or graphite flakes do not have any self-reacting orcohesive bonding capability.

Again, specifically and most significantly, these graphene oxidemolecules present in a GO gel state are capable of chemically bonding,linking, or merging with one another and getting integrated intoextremely long and wide graphene planes (e.g. FIG. 3(a)) when the gel isdried and heat-treated at a sufficiently high temperature for asufficiently long period of time. These graphene planes can run as wideas the filament thickness or diameter and they are parallel to oneanother. No individual graphene platelets or sheets are discernible;they have been chemically converted to chemically active or living GOmolecules that are fully linked and integrated chemically with oneanother to form a unitary body in the fiber axis direction, and thesegraphene planes appear to be chemically bonded with one another alongthe thickness-direction (or Z-direction). X-ray diffraction studies haveconfirmed that the d-spacing (inter-graphene plane distance) has beenrecovered back to approximately 0.3354 nm (with 0%-0.001% by weight ofoxygen) to 0.40 nm (with approximately 5.0-10% oxygen). There does notappear to be any gap between these graphene planes and, hence, theseplanes have been essentially merged into one big unitary body, which islike a graphene single crystal. FIG. 3(a) depicts an example of such ahuge unitary body. The formation process for such a unitary grapheneentity is further illustrated in FIG. 3(c).

The starting graphitic material to be heavily oxidized for the purposeof forming graphene oxide gel may be selected from natural graphite,artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbonmicro-bead, soft carbon, hard carbon, coke, carbon fiber, carbonnano-fiber, carbon nano-tube, or a combination thereof. The graphiticmaterial is preferably in a powder or short filament form having adimension lower than 20 μm, more preferably lower than 10 μm, furtherpreferably smaller than 5 μm, and most preferably smaller than 1 μm.

Using artificial graphite with an average particle size of 9.7 μm as anexample, a typical procedure involves dispersing graphite particles inan oxidizer mixture of sulfuric acid, nitric acid, and potassiumpermanganate (at a weight ratio of 3:1:0.05) at a temperature oftypically 0-60° C. for typically at least 3 days, preferably 5 days, andmore preferably 7 days or longer. The average molecular weight of theresulting graphene oxide molecules in a gel is approximately20,000-40,000 g/mole if the treatment time is 3 days, <10,000 g/mole if5 days, and <4,000 g/mole if longer than 7 days. The required gelformation time is dependent upon the particle size of the originalgraphitic material, a smaller size requiring a shorter time. It is offundamental significance to note that if the critical gel formation timeis not reached, the suspension of graphite powder and oxidizer (graphiteparticles dispersed in the oxidizer liquid) appears completely opaqueand heterogeneous, meaning that discrete graphite particles or flakesremain suspended (but not dissolved) in the liquid medium. As soon asthis critical time is exceeded, the whole suspension becomes opticallytranslucent or transparent (if sufficiently low GO contents), or browncolored, meaning that the heavily oxidized graphite completely loses itsoriginal graphite identity and the resulting graphene oxide moleculesare completely dissolved in the oxidizer liquid, forming a homogeneoussolution (no longer just a suspension or slurry).

It must be further noted that if the suspension or slurry, with atreatment time being shorter than the required gel formation time, isrinsed and dried, we would simply recover a graphite oxide powder orgraphite intercalation compound (GIC) powder, which can be exfoliatedand separated to produce discrete nano graphene platelets (NGPs).Without an adequate amount of a strong oxidizing agent and an adequateduration of oxidation time, the graphite or graphite oxide particleswould not be converted into the GO gel state.

If the graphene oxide gel is obtained from a graphitic material havingan original graphite grain size (e.g. an average grain size, D_(g)), theresulting unitary graphene material is a single crystal or apoly-crystal graphene structure having a grain size significantly largerthan this original grain size. The unitary graphene material does nothave any grain that can be identified to be associated with anyparticular particle of the starting graphitic material. Originalparticles have already completely lost their identity when they areconverted into graphite oxide molecules that are chemically linked upand merged or integrated into a network of graphene chains essentiallyinfinite in molecular weight.

Further, even if graphene oxide gel is obtained from a graphiticmaterial having multiple graphite crystallites exhibiting no preferredcrystalline orientation (e.g. powder of natural graphite) as determinedby an X-ray diffraction or electron diffraction method, the resultingunitary graphene material (a single crystal or a poly-crystal graphenestructure) typically exhibits a very high degree of preferredcrystalline orientation as determined by the same X-ray diffraction orelectron diffraction method. This is yet another piece of evidence toindicate that the constituent graphene planes of hexagonal carbon atomsthat constitute the particles of the original or starting graphiticmaterial have been chemically modified, converted, re-arranged,re-oriented, linked or cross-linked, merged and integrated,re-graphitized, and even re-crystallized.

Example 1: Preparation of Discrete Nano Graphene Platelets (NGPs)

Chopped graphite fibers with an average diameter of 12 μm and naturalgraphite particles were separately used as a starting material, whichwas immersed in a mixture of concentrated sulfuric acid, nitric acid,and potassium permanganate (as the chemical intercalate and oxidizer) toprepare graphite intercalation compounds (GICs). The starting materialwas first dried in a vacuum oven for 24 h at 80° C. Then, a mixture ofconcentrated sulfuric acid, fuming nitric acid, and potassiumpermanganate (at a weight ratio of 4:1:0.05) was slowly added, underappropriate cooling and stirring, to a three-neck flask containing fibersegments. After 16 hours of reaction, the acid-treated graphite fibersor natural graphite particles were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) was subjected to a thermal shock at 1050° C. for 45seconds in a tube furnace to form exfoliated graphite (or graphiteworms).

Five grams of the resulting exfoliated graphite (graphite worms) weremixed with 2,000 ml alcohol solution consisting of alcohol and distilledwater with a ratio of 65:35 for 12 hours to obtain a suspension. Thenthe mixture or suspension was subjected to ultrasonic irradiation with apower of 200 W for various times. After two hours of sonication, EGparticles were effectively fragmented into thin NGPs. The suspension wasthen filtered and dried at 80° C. to remove residue solvents. Theas-prepared NGPs have an average thickness of approximately 9.7 nm. NGPsare used here for comparison purposes since these sheets or plateletsare not living chains and, under comparable processing conditions, donot lead to graphitic fibers that exhibit the combined conductivity,strength, and modulus characteristics of the presently invented GOgel-derived unitary graphene fibers. These discrete, “dead” graphenesheets cannot get chemically linked with one another and the resultinggraphene fibers are found to be relatively brittle.

Example 2: Preparation of Graphene Oxide (GO) Gel

In one example, graphite oxide gel was prepared by oxidation of graphiteparticles with an oxidizer liquid consisting of sulfuric acid, sodiumnitrate, and potassium permanganate at a ratio of 4:1:0.05 at 30° C.When natural graphite (particle sizes of 14 μm) were immersed anddispersed in the oxidizer mixture liquid, the suspension or slurryappeared optically opaque and dark. The suspension remained opaqueduring the first 52 hours of reaction. However, the suspension graduallyturned optically translucent (a little cloudy) when the reaction timeexceeds 52 hours, and the color of the suspension changed from black todark brown. After 96 hours, the suspension suddenly became an opticallytranslucent solution with light brown color. The suspension was asolution, which appeared very uniform in color and transparency,indicating the absence of any dispersed discrete objects. The wholesolution behaves like a gel, very similar to a typical polymer gel.

Thin and narrow filaments of this GO gel were dispensed and deposited ona on a plastic sheet surface moving from one roller to another roller.By removing the liquid medium from the cast GO gel filaments we obtainedthin graphene oxide filaments. These thin fibers look like, feel like,and behave like a regular polymer fiber. However, upon re-graphitizationat a temperature (from 100° C., to 2,800° C.) for typically 1-5 hours,each GO fiber was transformed into a unitary graphene entity comprisinglarge-size graphene domains (e.g. FIG. 3(a)).

The X-ray diffraction curves of a GO filament (GO gel filament laid on aglass surface with liquid medium removed) prior to a heat treatment, aGO filament thermally reduced at 150° C. for one hour, and a highlyreduced and re-graphitized GO filament (a unitary graphene fiber) areshown in FIG. 7(a), FIG. 7(b), and FIG. 7(c), respectively. The peak atapproximately 2θ=12° of the dried GO filament (FIG. 7(a)) corresponds toan inter-graphene spacing (d₀₀₂) of approximately 0.7 nm. With some heattreatment at 150° C., the GO fiber exhibits the formation of a humpcentered at 22° (FIG. 7(b)), indicating that it has begun the process ofdecreasing the inter-graphene spacing, reflecting the beginning ofchemical linking and ordering processes. With a heat treatmenttemperature of 2,500° C. for one hour, the d₀₀₂ spacing has decreased toapproximately 0.336, close to 0.3354 nm of a graphite single crystal.

With a heat treatment temperature of 2,750° C. for one hour, the d₀₀₂spacing is decreased to approximately to 0.3354 nm, identical to that ofa graphite single crystal. In addition, a second diffraction peak with ahigh intensity appears at 2θ=55° corresponding to X-ray diffraction from(004) plane (FIG. 7(d)). The (004) peak intensity relative to the (002)intensity on the same diffraction curve, or the I(004)/I(002) ratio, isa good indication of the degree of crystal perfection and preferredorientation of graphene planes. The (004) peak is either non-existing orrelatively weak, with the I(004)/I(002) ratio <0.1, for all graphiticmaterials heat treated at a temperature lower than 2,800° C. TheI(004)/I(002) ratio for the graphitic materials heat treated at3,000-3,250° C. (e.g. highly oriented pyrolytic graphite, HOPG) is inthe range of 0.2-0.5. For instance, a polyimide-derived pyrolyticgraphite with a HTT of 3,000° C. for two hours exhibits a I(004)/I(002)ratio of about 0.41. In contrast, a unitary graphene filament preparedwith a HTT of 2,750° C. for one hour exhibits a I(004)/I(002) ratio of0.78 and a Mosaic spread value of 0.21, indicating a practically perfectgraphene single crystal with an exceptional degree of preferredorientation.

The “mosaic spread” value is obtained from the full width at halfmaximum of the (002) reflection in an X-ray diffraction intensity curve.This index for the degree of ordering characterizes the graphite orgraphene crystal size (or grain size), amounts of grain boundaries andother defects, and the degree of preferred grain orientation. A nearlyperfect single crystal of graphite is characterized by having a mosaicspread value of 0.2-0.4. Most of our unitary graphene materials have amosaic spread value in this range of 0.2-0.4 (if obtained with a heattreatment temperature no less than 2,000° C.).

It may be noted that the I(004)/I(002) ratio for all tens of flexiblegraphite and graphene paper samples investigated are all <<0.05,practically non-existing in most cases. The I(004)/I(002) ratio for allNGP paper/membrane samples and all the graphene fibers prepared throughthe coagulation route is <0.1 even after a heat treatment at 3,000° C.for 2 hours. These observations have further confirmed or affirmed thealready established notion that the presently invented unitary graphenefiber is a new and distinct class of material that is fundamentallydifferent from any pyrolytic graphite (PG), flexible graphite (FG), andpaper/film/membrane/fibers of conventional graphene/GO/RGOsheets/platelets (NGPs).

The inter-graphene spacing values of GO gel-derived unitary graphenefibers obtained by heat treating at various temperatures over a widetemperature range are summarized in FIG. 8(a). Corresponding oxygencontent values in the GO gel-derived unitary graphene filaments areshown in FIG. 8(b). In order to show the correlation between theinter-graphene spacing and the oxygen content, the data in FIG. 8(a) andFIG. 8(b) are re-plotted in FIG. 8(c). A close scrutiny of FIG. 8(a) toFIG. 8(c) indicates that there are four HTT ranges (100-600° C.;600-1,250° C.; 1,250-2,000° C., and >2,000° C.) that lead to fourrespective oxygen content ranges and inter-graphene spacing range.

It is of significance to point out that a heat treatment temperature aslow as 600° C. is sufficient to bring the average inter-graphene spacingin GO to below 0.4 nm, getting closer and closer to that of naturalgraphite or that of a graphite single crystal. The beauty of thisapproach is the notion that this GO gel strategy has enabled us tore-organize, re-orient, and chemically merge the planar graphene oxidemolecules from originally different graphite particles or graphenesheets into a graphene monolith with all the graphene planes now beinglarger in lateral dimensions (significantly larger than the length andwidth of original graphene planes) and essentially parallel to oneanother. This has given rise to a thermal conductivity already >420 W/mK(with a HTT of 500° C.) and >950 W/mk (with a HTT of 700° C.), which isalready greater than the value (884 W/mK) of K1100 graphite fibers(Amoco) that are known to have the highest thermal conductivity amongall continuous carbon/graphite fibers. The K1100 is obtained from afinal HTT of 3,000° C., but we are able to achieve a higher thermalconductivity at such a low re-graphitization temperature. This isastonishing.

These planar GO molecules are derived from the graphene planes thatconstitute the original structure of starting natural graphite particles(used in the procedure of graphite oxidation to form the GO gel). Theoriginal natural graphite particles, when randomly packed into anaggregate or “graphite compact”, would have their constituent grapheneplanes randomly oriented, exhibiting relatively low thermal conductivityand having essentially zero strength (no structural integrity). Incontrast, the strength of the unitary graphene layer is typicallyalready in the range of 0.5-8 GPa.

With a HTT as low as 800° C., the resulting unitary graphene filamentexhibits a thermal conductivity of 1,148 W/mK, in contrast to theobserved 252 W/mK of the graphene fibers via spinning-coagulation withan identical heat treatment temperature. As a matter of fact, no matterhow high the HTT is (e.g. even as high as 2,800° C.),coagulation-derived graphene fibers only shows a thermal conductivitylower than 600 W/mK. At a HTT of 2,800° C., the presently inventedunitary graphene layer delivers a thermal conductivity of 1,805 W/mK(FIG. 6(a)).

Scanning electron microscopy (SEM), transmission electron microscopy(TEM) pictures of lattice imaging of the graphene filament, as well asselected-area electron diffraction (SAD), bright field (BF), anddark-field (DF) images were also conducted to characterize the structureof unitary graphene fibers.

A close scrutiny and comparison of FIG. 3(a) and FIG. 2(c) indicatesthat the graphene planes in a unitary graphene fiber monolithic aresubstantially oriented parallel to one another; but this is not the casefor coagulation-derived graphene fibers. The inclination angles betweentwo identifiable layers in the unitary graphene entity are mostly lessthan 5 degrees. In contrast, there are so many folded graphene sheets,kinks, pores, and mis-orientations in coagulation-derived graphenefibers.

Examples 3: Electrical and Thermal Conductivity Measurements of VariousGraphene Oxide-Derived Unitary Graphene Fibers

Four-point probe tests were conducted on unitary graphene fibers andcoagulation-derived graphene fibers. Their in-plane thermal conductivitywas measured using a laser flash method (Netzsch Thermal DiffusivityDevice).

FIG. 5 (a) and FIG. 5(b) show the thermal conductivity and electricalconductivity values, respectively, of the GO gel-derived unitarygraphene-based continuous fibers and those of the fibers produced byspinning of GO suspension into a coagulation bath, all plotted as afunction of the final heat treatment temperature. These data haveclearly demonstrated the superiority of the unitary graphene-basedfibers in terms of the achievable thermal conductivity and electricalconductivity at a given heat treatment temperature. All the prior artwork on the preparation of continuous graphene fibers results in asimple aggregate or twisted stack of discrete graphene/GO/RGO sheets.These simple aggregates or stacks exhibit many folded graphene sheets,kinks, gaps, and mis-orientations and, hence, are not amenable tore-graphitization of these graphitic sheets or re-crystallization ofgraphitic domains, resulting in poor thermal conductivity, lowelectrical conductivity, and weak mechanical strength. As shown in FIG.5(a), even at a heat treatment temperature as high as 2,800° C., thecoagulation-derived graphene fibers exhibit a thermal conductivity lessthan 600 W/mK, much lower than the >1,800 W/mK of the GO gel-derivedunitary graphene entity.

Conductivity values from two high-conductivity graphite fibers (K-1100and P2 from Amoco) are also included for comparison purposes. K-1100,with a final HTT as high as 3,000° C., exhibits a thermal conductivity(K) of 885 W/mK and electrical conductivity of 7,407 S/cm. P2 fiber,with a final HTT of 2,650° C., exhibits a thermal conductivity (K) of661 W/mK and electrical conductivity of 5,525 S/cm.

By contrast, the presently invented unitary graphene fibers do not haveto go through an ultra-high-temperature graphitization treatment toachieve a high thermal conductivity (e.g. K already=903 W/mK withHTT=600° C. and K=1,487 W/mK with T=1,250° C.). Graphitization of acarbonized fiber (e.g. PAN fiber) requires a temperature typicallyhigher than 2,000° C., most typically higher than 2,500° C. Thegraphitization temperature is most typically in the range of2,800-3,200° C. in order for carbonized fibers to achieve a thermalconductivity of 600-885 W/mK. In contrast, the typical heat treatmenttemperature (re-graphitization treatment) of the presently inventedGO-coated laminates is significantly lower than 2,500° C., typicallylower than 2,000° C., and more typically lower than 1,500° (can be lowerthan 1,000° C. or even lower than 600° C.). Graphitization ofpitch-based carbon fibers at 2,650° C. gives rise to an electricalconductivity of 5,525 S/cm (P2 fiber). However, our GO-derived unitarygraphene fibers achieve 5,952 S/cm at a HTT of 1,500° C. Additionally,K-1100, with a final HTT as high as 3,000° C., exhibits an electricalconductivity of 7,407 S/cm. In contrast, we achieve 16,820 S/cm at2,800° C. with our graphitic fibers. For continuous fibers, a thermalconductivity of 1,805 W/mK and electrical conductivity of 16,820 areunprecedented. After 60 years of worldwide intensive research, the bestcarbon/graphite fibers do not even come close to these performancevalues.

The continuous unitary graphene fibers, the prior art carbon/graphitefibers, and prior art graphene fibers are three fundamentally differentand patently distinct classes of materials in terms of chemicalcomposition, morphology, structure, process of production, and variousproperties.

Examples 4: Tensile Strength of Various Graphene Oxide-Derived UnitaryGraphene-Based Fibers

A series of GO gel-derived unitary graphene fibers were prepared. Auniversal testing machine was used to determine the tensile strength andYoung's modulus of these materials. FIG. 6(a) and FIG. 6(b) summarizethe tensile strength and Young's modulus of the GO gel-derived unitarygraphene-based continuous fibers plotted as a function of the final heattreatment temperature. In FIG. 6(c), tensile strength values are plottedas a function of the Young's modulus of the same fibers.

These data have demonstrated that, the tensile strength and Young'smodulus of the GO-derived unitary graphene fibers have exceeded thehighest strength and highest modulus ever achieved by any continuouscarbon or graphite fiber. It may be noted that the carbon/graphitefibers exhibiting the highest tensile strength are derived fromPAN-based polymer fibers, but the carbon/graphite fibers exhibiting thehighest tensile Young's modulus are derived from petroleum pitch. Inother words, highest tensile strength and highest Young's modulus couldnot be achieved with the same type of carbon/graphite fibers. This is incontrast to the presently invented graphitic fiber, which achieves boththe highest tensile strength and the highest Young' modulus with thesame fiber. The GO-derived unitary graphene fibers are a class ofmaterial by itself.

We claim:
 1. A process of producing a fabric comprising at least oneunitary graphene-based continuous graphitic fiber, said processcomprising: a) preparing a graphene oxide gel in a fluid medium; b)depositing at least a continuous filament of graphene oxide gel; c)removing said fluid medium to form a continuous graphene oxide fiber; d)heat treating said continuous graphene oxide fiber to form a unitarygraphene-based continuous fiber; e) forming a continuous graphitic yarncomprising at least one of said unitary graphene-based continuous fiber;and f) creating a fabric containing said continuous graphitic yarn;wherein said unitary graphene-based continuous graphitic fiber has aporosity level less than 5% by volume, an oxygen content less than 5% byweight.
 2. The process claim 1, wherein said depositing step may beconducted via a procedure selected from coating, casting, injection,extrusion, pultrusion, roller, doctor blade, wiper or spinning of thegraphene oxide gel onto a solid substrate along a fiber axis direction.3. The process of producing the fabric comprising at least one unitarygraphene-based continuous fiber of claim 1, wherein said depositing stepmay be conducted under a condition of mechanical stress.
 4. The processclaim 1, wherein said depositing step is onto a substrate.
 5. Theprocess of claim 1, wherein said continuous filament of graphene oxidegel has a cross-section that is circular, elliptical, rectangular,flat-shaped, or hollow.
 6. The process claim 1, where said heattreatment temperature is greater than 600° C.
 7. The process of claim 1,where said heat treatment temperature is greater than 1250° C.
 8. Theprocess of claim 1, where said heat treatment temperature is greaterthan 2000° C.
 9. The process of claim 1, where said heat treatment takesplace in a stress field that includes a local tension stress along afiber axis direction.
 10. The process of claim 1, where said heattreatment induces chemical merging of individual graphene oxide in anedge-to-edge manner.
 11. The process of claim 1, further comprisingchemical functionalization of said graphene oxide, said continuousgraphene oxide fiber, or said dried continuous graphene oxide fiber. 12.The process of claim 1, further comprising a step of combining multiplefilaments together to create said continuous graphitic yarn.
 13. Theprocess of claim 12, wherein said continuous graphitic yarn furthercomprises at least one fiber selected from the group consisting of wool,cotton, asbestos, nylon, synthetic, carbon nanotubes, and graphene-basedgraphitic fiber.
 14. The process of claim 1, wherein said fabricproduction step is a weaving process and said unitary graphene-basedcontinuous fiber is contained in at least warp or weft.
 15. A fabricmade by a process of producing a fabric comprising at least one unitarygraphene-based continuous graphitic fiber, said process comprising: a)preparing a graphene oxide gel in a fluid medium; b) depositing at leasta continuous filament of graphene oxide gel; c) removing said fluidmedium to form a continuous graphene oxide fiber; d) heat treating saidcontinuous graphene oxide fiber to form a unitary graphene-basedcontinuous fiber; e) forming a continuous graphitic yarn comprising atleast one of said unitary graphene-based continuous fiber; and f)creating a fabric containing said continuous graphitic yarn; whereinsaid unitary graphene-based continuous graphitic fiber has a porositylevel less than 5% by volume, an oxygen content less than 5% by weight.16. The fabric of claim 15, wherein said fabric has a volumetric packingdensity greater than 20% and less than 90%.
 17. The fabric of claim 15,wherein said continuous graphitic yarn has a cross-section that isrectangular or flat-shaped, having a width and a thickness.
 18. Thefabric of claim 17, wherein said yarn has a width-to-thickness ratiogreater than
 5. 19. The fabric of claim 15, wherein said at least one ofsaid unitary graphene-based continuous fibers contains chemicalfunctionalization.
 20. The fabric of claim 15, wherein said fabric iselectrically conductive.
 21. A heating device containing the fabric ofclaim 20, wherein said fabric acts as a resistance heater when anelectric current is applied.
 22. The fabric of claim 15, wherein saidfabric is configured to block pathogenic agents.